Mike Farrell ( born February 6 , 1939 ) is an American actor and activist . He played BJ Hunnicutt on the TV show , M \* A \* S \* H. In recent years he has become an activist for various causes , like animal rights and world peace . Michael Joseph `` Mike '' Farrell ( born February 6 , 1939 ) is an American actor , best known for his role as Captain B.J. Hunnicutt on the television series M \* A \* S \* H ( 1975 -- 83 ) .