We have previously reported that treatment with a unique lymphokine enables resident mouse peritoneal macrophages to phagocytize via their complement receptors and we have presented evidence that the lymphokine act by enabling complement receptor engagement by C3b ligands to generate a phagocytic signal, thereby linking the cell surface binding event with the intracellular phagocytic machinery. In the present experiments, we used immobilized immune complexes to study the topography of C3b receptors of resident mouse peritoneal macrophages treated with the lymphokine. Our results indicate that lymphokine treatment enables the macrophages' C3b receptors to migrate within the plane of the cells' plasma membrane and that manipulations of macrophages that abrogate one response to the lymphokine, complement receptor mobility, also abrogate the other response, complement receptor-mediated phagocytosis. These findings strongly suggest that lateral mobility of a ligand-bound receptor within the macrophage plasma membrane is an essential component of the phagocytic signal. Moreover, our results indicate that the difference in complement receptor function among various populations of macrophages is not due to the expression of different types of complement receptors by the different macrophage populations but rather to a difference in the relationship of the C3b receptor with other plasma membrane or intracellular components.