The rat monoclonal antibody (ab) 7/4 produced against neutrophil-rich cultured bone-marrow populations defines a polymorphic neutrophil differentiation antigen (ag). Ag 7/4 expression was characterized on cells from C57PL/6 and Swiss PO outbred mice using fluorescence-activated cell sorter (FACS) analysis and radioimmune binding assays. All neutrophils in bone marrow; blood, and inflammatory exudates were labeled by ab 7/4 and appeared to express similar amounts of ag 7/4 quantitated by saturation radioimmune binding assays. However, discrete populations of dimly (8–12% of nucleated cells) and brightly (30–40%) 7/4 labeled cells could be resolved by FACS analysis of bone marrow only. No binding was detected to resident or inflammatory macrophages, lymphocytes, eosinophils, mast cells and mature erythroid cells in hemopoietic or lymphoid tissues.

Nine inbred mouse strains tested expressed high levels of ag 7/4, while six other strains expressed insignificant levels. The expression of ag 7,/4 on bone-marrow cells from F₁, F₂, and backcross generation mice was consistent with control by a single, autosomal dominant gene.