Swiss albino male mice were given Protein A (PA) treatment ip (0, 1.0, 6.0 or 12.0 micrograms per mouse) twice weekly for two weeks. Alterations in white blood cell counts, peritoneal cell number, peritoneal macrophage and mast cell number were found. Macrophage induced phagocytosis of sheep red blood cells was also found enhanced in PA group. The maximum effect was found in 1.0 microgram of PA dose group of mice. From our studies it can be derived that PA induced tumor regression may be due to potentiation of macrophage induced antitumor activity.