We determined the antigenic specificity and protective immunogenicity of two chemically synthesized peptides of type 5 streptococcal M protein. The synthetic peptides, designated S-M5(1-20) and S-M5(20-40), represent the amino-terminal amino acid sequence of the native pepsin-extracted M5 molecule, which is known to contain at least one heart cross-reactive epitope. Initial studies showed that neither of the synthetic peptides was able to bind purified heart-reactive M5 antibodies. In addition, S-M5(1-20), but not S-M5(20-40), contained type-specific antigenic determinants as measured by enzyme-linked immunosorbent inhibition assays. When covalently linked to tetanus toxoid, S-M5(1-20), but not S-M5(20-40), evoked significant levels of type-specific, opsonic (and presumably protective) antibodies in rabbits without evoking heart cross-reactive antibodies.