Streptococcal M protein was studied to ascertain its effect on macrophage functions such as oxidative metabolism, phagocytosis, and microbicidal activity. No significant stimulating activity of M protein against macrophage functions such as H2O2 or O2- releasing ability in response to phorbol myristate acetate triggering was noted. M protein did not enhance the macrophage-mediated host risistance to infection with Listeria monocytogenes. However, macrophages elicited with M protein showed a somewhat more enhanced phagocytic activity than did the resident macrophages.