The phagocytosis of uniform fluorescent latex particles by resident and thioglycollate-elicited macrophages was analysed by flow cytometry. The percentage of phagocytosing macrophages and the number of internalized microspheres per cell was determined from cell size and fluorescence histograms. Results were corrected for the adherence of microbeads to the cells in the presence of sodium azide in the medium. Human C3b- or murine monoclonal IgG-coated microspheres were applied to assess receptor-mediated phagocytosis in different inbred strains of mice. Phagocytic activity of thioglycollate-elicited macrophages was consequently higher than that of resident macrophages. A decreasing gradient of C3b and Fc receptor-mediated phagocytosis was established in the following order: B10.BR, B10, C3H/Di and C3H.SW strains. Our results indicate that the phagocytic function of murine macrophages is under control of both the somatic (non-H-2) and H-2 genes.