There is evidence suggesting that Sydenham's chorea, which is a major manifestation of acute rheumatic fever, may be mediated by streptococcal antibodies that cross-react with the brain. Our studies were undertaken to determine whether streptococcal M protein, the major virulence factor of group A streptococci, evoked antibodies that cross-react with human brain. Rabbits were immunized with pepsin-extracted M protein from rheumatogenic type 6 streptococci. Immune sera were screened for the presence of antibodies that cross-reacted with human brain by indirect immunofluorescence tests and immunoblot analyses. Type 6 M protein evoked antibodies that cross-reacted with several brain proteins and antibody binding to these proteins was completely inhibited by type 6 M protein and partially inhibited by types 5 and 19 M proteins, suggesting that these heterologous M proteins contain conserved brain-cross-reactive epitopes. Using synthetic peptides from several serotypes of M proteins, the conserved brain-cross-reactive epitopes were localized to a decapeptide contained within the covalent structure of the B repeat region of type 6 M protein. These peptides also inhibited brain-cross-reactive antibodies in the serum of a patient with active Sydenham's chorea. Our data indicate that streptococcal M proteins contain brain-cross-reactive epitopes that could potentially be involved in the pathogenesis of Sydenham's chorea.