A mouse monoclonal antibody (HC1/1) specific for a differentiation marker of human monocytes and granulocytes has been generated by using as immunogen the monocytic cell line U937 differentiated with phorbol esters. This differentiation antigen has been characterized as the p150/95 member of the CD11 family of glycoproteins by cellular distribution studies, immunoprecipitation and competition experiments with MAb 3.9 specific for the CD11c antigen. Immuno-alkaline phosphatase staining of normal tissue sections with the HC1/1 MAb demonstrated that the CD11c antigen is a useful macrophage marker. Furthermore, the MAb HC1/1 stains specifically populations of macrophages on skin and lymph node sections from different pathologies such as Sarcoidosis, Granuloma annulare, Lepromatous leprosy and Toxoplasmosis.