Expression of F4/80 and Mac-1 (CD11b) antigens by ZK1 cells, as determined by flow-cytometry. Cell surface expression of the macrophage-associated differentiation Ags was assessed by direct immunofluorescence (thick lines). ZK1 cells were incubated with FITC-labeled anti-mouse F4/80 or FITC-labeled anti-mouse CD11b. Mouse IgG2a and IgG2b were used as isotype controls (light lines). Staining of cells with FITC-labeled anti-mouse Ig compared to unstained cells detects the % of cells expressing surface Ig. More than 99% of ZK1 cells express F4/80 and CD11b antigens.