HMA-1 and HMA-2 are two serologically defined alloantigens that are present on human monocytes and granulocytes. Previous panel studies suggested that these two antigens comprise a diallelic system. We therefore performed segregation studies on 10 randomly chosen families and on 22 nucleus families belonging to one large pedigree. All individuals were either positive for HMA-1, HMA-2, or both. Segregation of HMA-1 showed autosomal inheritance with complete penetrance. The cytotoxicity negative, absorption positive (CYNAP) phenomenon sometimes occurred for HMA-2, and because absorptions for HMA-2 could not be performed on all cells, linkage studies were only performed for HMA-1. No linkage of HMA-1 with HLA, immunoglobulin allotypes, granulocyte antigens, or any of the erythrocyte blood groups tested was observed.