Human peripheral blood leukocytes were incubated with thirteen various merocyanines of the stilbazolium betaine type and the fluorescence intensities of the cells were measured by flow cytometry. The fluorescence intensity of lymphocytes, monocytes and granulocytes depended on the time and temperature of incubation with the dyes. An increase in the incubation temperature enhanced the fluorescence intensity whereas washing of the cells after incubation had little influence on the observed emission. This points to incorporation of the dye molecules into the cell membrane. From the measured fluorescence intensities corrected for relative fluorescence yields, the relative efficiencies of incorporation into the cells of the various merocyanines tested were evaluated. The efficiency was dependent on the type of the cells and the lenght and side groups of the merocyanine molecules studied.