The susceptibility of HL-60 cells to the cytotoxic activity of leukocidin increased depending on the degree of differentiation induced by dimethyl sulfoxide (DMSO). To compare binding characteristics of two components (S and F) of leukocidin to HL-60 and DMSO-treated HL-60 cells, the S and F components were labeled with 125I. Scatchard analysis of the binding curve of the 125I-labeled S component to HL-60 cells showed two classes of binding sites. The binding sites with higher affinity had a dissociation constant of 3.39 nM, and the number of binding sites per cell was 730. The specific binding of the 125I-labeled S component to DMSO-treated cells increased depending on the period of DMSO treatment. Scatchard analysis of the binding curve of cells treated with DMSO for 7 days gave a straight line. The dissociation constant was 1.78 nM, and the number of binding sites per cell was 6,920. The total binding of the 125I-labeled F component to DMSO-treated cells increased about twofold over binding to HL-60 cells. However, in the presence of the unlabeled S component, the increase of binding of the F component to DMSO-treated cells was much greater. These data suggested that the increased susceptibility of DMSO-treated cells to leukocidin was dependent on the changes in the number of high-affinity binding sites of the S component and of the bound F component.