One of 19 mAbs against the native group A streptococcal M6 protein proved opsonic for type 6 organisms in a bactericidal assay. The opsonic and three nonopsonic antibodies were selected for isotype and complement fixation studies based on previous knowledge of their epitope site on the M6 molecule. While mAb 3B8 (IgG3), whose epitope is in the NH2-terminal hypervariable region of the molecule (distal from the cell), and mAbs 10B6 (IgG2a) and 10F5 (IgG2b), both located in the conserved central region of the molecule, all fix complement, 10A11 (IgG1) did not. Only mAb 3B8 was opsonic despite the fact that mAbs 10B6 and 10F5 both exhibited similar complement-fixing capacity, binding titer, and surface exposure of epitopes. Analysis of antibodies raised against synthetic peptides representing various regions of the M6 protein showed that only the amino-terminal peptide (residues 1-21) was capable of eliciting opsonic antibodies, despite the fact that peptides from other areas produced antibodies with high-binding titers to the native M6 protein and also with the ability to bind to intact streptococcal cells. These results not only support the observed type specificity of opsonic antibodies, but also clearly point to the importance of the location of antibody binding on the M molecule relative to the actual functional capacity of the antibody with respect to the opsonization and phagocytosis of M6 streptococci. These results may underscore the recently observed role of complement Factor H in the antiphagocytic activity of the M protein.