M proteins project as a-helical coiled-coil fibrils from the cell surface of Streptococcus pyogenes (1,2). These surface fibrils render the organisms resistant to ingestion and killing by blood phagocytes (3). The antiphagocytic effect is neutralized by type-specific antibodies directed against the M protein. In addition to the type-specific antibodies, certain M proteins evoke crossreactive immune responses against sarcolemmal membranes of cardiac tissue (4, 5) and muscle myosin (6). The latter findings have hampered efforts to develop M protein vaccines against rheumatogenic strains of S. pyogenes because of the fear that the vaccines may trigger rather than prevent acute rheumatic fever and rheumatic heart disease. In an attempt to avoid heart crossreactive epitopes, we prepared vaccines from native and chemically synthesized subpeptide fragments of M protein. We found that selected synthetic peptides copying known amino acid sequences of different M proteins evoked type-specific protective immunity against the related streptococci without stimulating heart crossreactive immune responses (7-13). In this paper, we report that hybrid peptides containing copies of selected regions of two different serotypes of M protein synthesized in tandem evoke protective but not heart crossreactive immune'responses against both serotypes of S. pyogenes. Our findings may have bearing not only on the development of safe, multivalent M protein vaccines to protect against the many M serotypes of streptococci giving rise to acute rheumatic fever and rheumatic heart disease, but also on the development of multivalent hybrid vaccines against a variety of other infectious agents. Materials and MethodsNatural and Synthetic Peptides of Streptococcal M Protein. Natural polypeptide fragments of M protein were isolated and purified from limited digests of types 5 and 24 M proteins