Feel like I'm fixin' for a comeback ...

"And it's one, two, three, what are we fighting for? Don't ask me, I don't give a damn, next stop is Vietnam." It's amazing how easily the words slip back into the memory, like suddenly discovering a forgotten tie-dye vest at the back of the wardrobe and finding that it still fits. It's even more amazing when you consider that more than three decades have passed since the song "Feel Like I'm Fixin' to Die Rag" first appeared, not to mention all the drugs that have been consumed in the intervening years (by us, the listeners, that is, not Country Joe, who at 56 still seems as clear-eyed and as optimistic as he did at Woodstock). It was the movie Woodstock that granted McDonald his true 15 minutes of fame, recording the moment when he got the audience to sing along with his legendary Fish cheer, although of course he changed the "fish" into a "fuck". "Give me an F! Give me a U! Give me a C! Give me a K! What's that spell?", he bellowed, to be answered by the loudest mass expletive in history. And then he sang "I Feel Like I'm Fixin' to Die Rag", with its immortal line: "Be the first family on your block to have your boy come home in a box."

According to McDonald, it almost never happened. "It was all a mistake," he says. "I was just hanging around the stage, Richie Havens had finished, and there was nothing going on. I went on with my guitar and it was like, `Here's this guy who's going to sing', but no one paid any attention. I played `Janis' and `Tennessee Stud' and then I walked off stage. I asked my tour manager if he thought it would be OK if I went back on and did the cheer and he said yeah. So I went "Give me an F!", and they all yelled "F!". A month or so later Michael Wadleigh [the director of Woodstock] showed me the footage in this little studio in LA, and after that it kind of became my fate. It's not exactly what I wished my musical career to be, but it happened. I hated it for a long time, because you can't think about me without thinking about the Vietnam war, but later I met a lot of Vietnam vets and they told me they'd sung the song in Vietnam, and now I'm involved in doing a lot of healing work with veterans' groups."

Tomorrow, 13 May, the song and the cheer may well be heard again, when Country Joe McDonald plays his first British show for 20 years at the Queen Elizabeth Hall in London. What McDonald is really excited about, however, is not the concert, but Florence Nightingale. The date of the concert provides him with a chance to attend two ceremonies commemorating the anniversary of Nightingale's birth in 1820 - at the family grave in Hampshire, and again at Westminster Abbey. "I've been interested in her for about 17 years, and I started researching her life," he says. "I've been to the Crimea and to the hospital in Turkey where she worked, and also visited her summer home in England."

This, on the face of it surprising, enthusiasm began as part of McDonald's continuing involvement with the cause of Vietnam veterans. "I was at a conference in 1981 and a nurse from Vietnam accused us all of denying the role of women in war. As I'm a military veteran myself, I decided to look into it. I found the two volumes of Sir Edward Cook's biography of Florence Nightingale, and learned about her experiences in the Crimean war. It seemed to me that the trauma she suffered afterwards was similar to a kind of delayed stress syndrome."

McDonald's own military experience began when he volunteered for the navy as a 17-year-old in 1959. "I had just graduated from high school and it seemed like a good idea, an attempt to have sex and see the world. It was also to try to clear the family name, because my parents were Communist Party members and we had been accused of being unpatriotic. In the long run it all turned out OK, except that girls didn't like soldier's haircuts. I was honourably discharged after serving in Japan for two years with Air Traffic Control. Pilots would keep requesting maps for Vietnam, but I didn't know anything about the conflict then."

Although McDonald is remembered perhaps most of all for his band, the endearingly eccentric Country Joe and the Fish - one of the first and the best of the San Francisco psychedelic groups, whose guitarist Barry Melton's long, liquid lines helped define the sound of the era - his most famous song pre-dates them by a couple of years. "Feel Like I'm Fixin' to Die Rag" was first recorded in Berkeley in 1965 as a kind of skiffle song. "I was writing music for an anti-military play and I picked up the refrain from an old Dixieland trombone riff on `Muskrat Ramble'," McDonald says. "The lyrics came from my parents having radical socialist beliefs, and from me coming out of the military. It was written in 20 minutes, and within 60 days we'd recorded it and were selling it at a Teach-In at 50 cents for a 7-in EP. I had met Barry Melton in Berkeley in 1965 and over the next 12 months minimal electric instruments were added to the skiffle thing, and then it quickly transformed itself into the Fish, with songs like `Bass Strings'."

The group's first two albums, "Electric Music for the Mind and Body", and "I Feel Like I'm Fixin' To Die" (which featured a rerecorded version of the title song), were wacky even by the standards of the time, mixing mordant social commentary with trance-like tunes that offered listeners a sympathetic soundtrack for their own psychedelic experiences. Now McDonald's trance music has found a new audience among the post-rave generation, and his London concert should feature an appearance by the group the Bevis Frond, whose leader, Nick Solomon, he met recently at a San Francisco festival of 38 psychedelic trance bands.

"I've been totally unsuccessful in trying to get the original Country Joe and the Fish together again, so I've pretty much accepted the fact," McDonald says regretfully. "Barry Melton is a lawyer and a part-time musician now and he's not into playing with the other original members, though he'll still play with me as a duo."

And what, I ask, of the group's legendary drummer, Chicken Hirsch, whose Woody Allen-like appearance always looked wonderfully out of sync on album sleeve photos? "He's in Oregon now, and big in the T-shirt business," McDonald says. "The band was like a family, and that was the biggest enjoyment I got in music."

McDonald has plenty of reasons to get misty-eyed about the past. He once lived with Janis Joplin, and when Jerry Garcia died, he took it as a personal blow. "When Jerry died I quit for a year, the only time in my life I didn't play music", he says. `I just packed my guitar up and became a housewife. I've got five kids from three different marriages; the oldest is 30 in a month and the youngest is 6. After a year I got back into it through computers and web-sites, but I don't want to die on the road like Jerry. I want to have fun."

It doesn't seem the moment to mention the refrain of his most famous song: "Whoopee we're all going to die!"

Country Joe McDonald plays the Queen Elizabeth Hall, London, on 13 May. To coincide with the concert, Ace Records are releasing a compilation album, `Something Borrowed, Something New', and reissuing his 1970 film soundtrack, `Quiet Days in Clichy'.