In the high-responder A/J mice a lower percentage of peritoneal macrophages express Fc receptors and accordingly phagocytize less intensely via these receptors than the macrophages of the low-IgG-responder C57BL/10ScSn strain. Similarly, the phagocytosis mediated by the receptors for foreign substances tested by CdCO3 microcrystals is lower in the A/J strain. Pinocytic activity was the same in both mouse strains. Intraperitoneal immunization with sheep red blood cells led to a prompt increase in Fc receptor expression and in both phagocytic and pinocytic activities in A/J mice, while in C57BL/10ScSn mice these activities remained unchanged. A similar effect was found when spontaneous and induced rosette formation with sheep red blood cells was studied. The opposite was true when the phagocytic activity of peripheral leucocytes to non-antigenic synthetic microspheres was studied; after immunization with sheep red blood cells a strong increase was observed not in the A/J but C57BL/10ScSn strain. The importance of these results for the regulatory role of macrophages in immune responses in the low- and high-IgG-responder strains of mice is discussed.