Streptococcal M protein, the antiphagocytic molecule on the surface of the organism, was previously found to exhibit extensive size heterogeneity between as well as within M serotypes. In this study, methods were devised to isolate M protein size mutants within a laboratory-grown culture. We were able to isolate three independent M protein deletion mutants and one additional mutant, which was derived from the first deletion mutant. We found that these deletion mutants occur at a frequency of approximately 1 in 2 X 10(3) CFUs in culture. Functional studies revealed that the deletion mutants were able to survive as well as the parental strain in human blood. They also had the determinants necessary to absorb opsonic antibodies as well as the parent. Pepsin digestion experiments localized the deletions within the N-terminal half of the M molecule, which is distal to the cell wall surface. This is the region of the molecule in which extensive sequence repeats are found. This is consistent with the suggestion that the size changes may be the result of homologous recombination between the repeat regions in the gene. These results support the idea that strains showing M protein size variation within successive clinical isolates from single patients may be derived from the initial infecting organisms, and are not the result of separate unrelated acquisitions of the same serotype. This size change may be important in the survival of the streptococcus in vivo.