Binding and phagocytosis of rat erythrocytes by liver and peritoneal macrophages were studied with a radioactive in vitro assay which yields quantitative data. Partial removal of sialic acids from the erythrocytes by Vibrio cholerae sialidase resulted in a marked increase of binding of the red cells by both liver and unstimulated peritoneal macrophages. Peritoneal macrophages stimulated by thioglycolate or starch, however, did not differentiate between control and desialylated erythrocytes. By inhibition experiments it was confirmed that rat peritoneal macrophages bind homologous sialidase-treated erythrocytes via a beta-D-galactose-specific lectin on the macrophage surface. While this attachment already occurs in buffer, serum was required for the subsequent phagocytosis. The possible involvement of factors of the complement system in the phagocytosis step was evidenced by a marked decrease of phagocytosis after heat inactivation of the serum. Based on these experiments, we propose a model of a two-step mechanism for the uptake of sialidase-treated erythrocytes by macrophages, comprising both the lectin and a receptor for serum components.