We have studied the effect of infection with the blood-stage of Plasmodium yoelii 17X, a nonlethal parasite, on plasma membrane antigens, receptors, and secretory properties of macrophages (M phi) in murine liver, spleen, and blood. mAb F4/80 (M phi specific), F7/4 (a marker for immature and immunologically activated M phi, as well as neutrophils), and Mac-1, which binds to the type 3 complement receptor, were used to measure the distribution and total content of antigens in situ and to assay surface expression of antigens on M phi isolated by collagenase perfusion-digestion and adherence. We also examined respiratory burst activity after stimulation with PMA, FcR activity, Ia antigen expression, and binding of 125I-mannose-BSA and unopsonized sheep erythrocytes by isolated M phi. In the normal animal, spleen M phi expressed Mac-1 and F7/4 antigens and relatively high levels of respiratory burst activity, in contrast to Kupffer cells in liver, where all three features were virtually absent. The introduction of parasitized erythrocytes into the circulation resulted in a large influx of F4/80+ M phi into the blood, liver, and spleen, where local M phi proliferation could also contribute. Liver M phi during malaria infection showed increased Mac-1 and 7/4 antigen and an increased respiratory burst potential compared with uninfected controls. Increases in total, but not specific activity of FcR, Ia antigen, and binding of unopsonized sheep erythrocytes were found in spleen and liver M phi populations after infection. In both populations, there was an early but persistent marked reduction in specific binding and uptake of 125I-mannose-BSA. These results confirm and extend observations that normal Kupffer cells are relatively homogeneous in morphology, surface markers, and anatomical location, in contrast to M phi in normal spleen, and that both of these populations differ from resident M phi elsewhere, including the peritoneal cavity. In the course of infection by P. yoelii, M phi with high levels of opsonic receptors (CR3, FcR) and respiratory burst potential are mobilized in large numbers at specific sites such as liver and spleen, in accordance with an important role for M phi in the clearance of parasitized erythrocytes from blood.