Expression of Fas antigens on ATL cells and neutrophils from patients with ATL. Heparinized blood was incubated with monoclonal antibody ZB4 (against Fas antigen) or control mouse IgG. Expression of Fas antigen on ATL cells and neutrophils was examined simultaneously by flow cytometry. Shadowed peaks represent Fas antigen and open peaks represent control mouse Ig. Case no. 1, Fas-positive ATL cells; case no. 2, Fas-negative ATL cells; case no. 3, ATL cells with decreased expression of Fas antigen.