Treatment of HL-60 cells with DMSO caused cells to differentiate along the myeloid way and 84% of the cells showed a reduction in NBT after 120h. Protein kinase C activity increased of during differentiation. It would reach 2.7 times that of the control after 72h in the presence of DMSO. The increase of protein kinase C activity was traced to the cytosolic rather than the membrane fraction. Chelator EGTA was necessary to extract protein kinase C. DMSO itself could not enhance protein kinase C activity, indicating that the effect of DMSO was indirect. All of these results showed that the activation of protein kinase C is important in the differentiation of HL-60 cells.