In this study, we utilized recombinant strains expressing the M5 and M18 proteins and M- mutants of group A streptococci to compare the abilities of these M proteins to confer resistance to phagocytosis and to mediate adhesion to host cells, The data indicate that the M5 and M18 proteins can confer resistance to phagocytosis, that fibrinogen is required for this resistance, and that these: M proteins can mediate adhesion to HEp-2 cells.