An electroimmunoblot technique was used to detect various forms of myelin basic protein (MBP) in brain homogenates of 14 vertebrate species. Three antibodies were used to probe the immunoblots: a monoclonal anti-human MPB reacting with an antigenic determinant located at amino acid residues 131 to 136; a polyclonal anti-human MBP and a polyclonal anti-chicken MBP. Because no processing of the tissue is required prior to electrophoresis, in vitro artifacts are minimized. The 18.5 K form of MBP was present in all species except the shark. A 21.5 K MBP was observed in ovine, bovine, pig, rabbit, mouse, rat, monkey, but not in human, guinea pig, shark, toad and marsupial brains. A variant with a molecular weight between 17 K and 18 K was found in mouse, rat, bovine, human, monkey, pig, and chicken brains, and was the sole component in the shark brain. Marsupial brains had five or six forms of MBP between 14.5 K and 18.5 K.