The ability of chemically synthesized subpeptides of type 24 streptococcal M protein to evoke protective antibodies in rabbits was investigated. We synthesized copies of the COOH-terminal 13, 18, and 23 amino acid residues of cyanogen bromide fragment 7 (CB7) of pepsin-extracted type 24 M protein, except that methionine was substituted for homoserine as the COOH-terminal residue. An additional residue of cysteine was added at the COOH terminus of the 13-residue peptide. Each of the peptides, designated S-CB7-(23-35)-Cys, S-CB7-(18-35), and S-CB7-(13-35), when conjugated to lysylated tetanus toxoid with glutaraldehyde, was capable of stimulating formation of protective anti-type 24 M protein antibodies in rabbits. The smallest peptide, S-CB7-(23-35)-Cys, elicited immune responses equally as strong, if not stronger, than those to the longer peptides. A single Lys/Gly substitution in this 13-residue peptide resulted in its failure to stimulate protective antibodies. None of the antisera reacted with heterologous serotypes of M protein and none reacted with frozen sections of human heart tissue. These results indicate that a chemically synthesized peptide fragment corresponding to as few as 13 amino acid residues of streptococcal M protein is capable of evoking protective anti-streptococcal antibodies without evoking antibodies crossreactive with cardiac tissue.