The mouse monoclonal antibody MN-41 has been characterized as an anti-human iC3b receptor (CR3) antibody on the basis of its ability to inhibit the binding of EC3bi indicator cells to monocytes and polymorphonuclear cells while having no effect on their Fc and C3b receptors. Use of this monoclonal antibody in indirect immunofluorescence studies with dual fluorochrome labels established the widespread distribution of CR3 in man--detected on 97% of circulating monocytes, 90% of granulocytes, 17% of T lymphocytes, and 28% of B lymphocytes while erythrocytes and platelets were negative. Isolated peritoneal macrophages were 90% positive while pulmonary macrophages were 83% positive. Monocytes in culture for 8 days were universally positive. Within tissues, CR3 reactive cells displayed unique topographical localization within the spleen, tonsil, and lymph nodes whereas numerically fewer positive cells were scattered within hepatic sinusoids, papillary dermis, medullary regions of the thymus, and submucosa of the small intestine. CR3 was not detected on Raji cells, glomerular epithelial cells, or placental stromal cells. Immunoprecipitation and electrophoretic separation of two glycoprotein bands of 150,000 and 95,000 Da suggest possible structural homology of CR3 in man and mouse (Mac-1 antigen).