The roles of lipoteichoic acid (LTA) and M protein in the adherence of group A streptococci to human cells were investigated. Both M+ and M- streptococci bound to pharyngeal and buccal epithelial cells in similar numbers. Streptococcal attachment was inhibited by LTA, but not by the pepsin-extracted, amino-terminal half of M protein (pep M), suggesting that M protein does not mediate attachment to these cells. However, a purified, recombinant, intact M protein did block attachment of streptococci to buccal cells. Using synthetic peptides, the inhibitory domain was localized to a region of intact M protein that is within or near the bacterial cell wall. Evidence is presented to suggest that on the surface of streptococci this region of the M protein is probably not accessible for interactions with host cell receptors and that M protein does not mediate attachment to buccal or pharyngeal cells. In contrast, approximately 10-times more M+ streptococci bound to Hep-2 cells than did M- streptococci and pep M protein blocked binding of streptococci to Hep-2 cells. The data suggest that at least two streptococcal adhesins, LTA and M protein, are involved in the adherence of streptococci to certain cells and that the relative contributions of these adhesins to the attachment process depends on the type of host cells used to study adherence.