When the leukemia cells in 14 patients with acute promyelocytic leukemia (APL) were induced by all-trans retinoic acid (RA) treatment, the activity of protein kinase C (PKC) increased with the differentiation. In the control cells, the activity of PKC was only 47 +/- 39.3 pmol.mg-1/min, while it was 149.3 +/- 150.1 pmol.mg-1/min after differentiation. There were 90.1 +/- 7.2% promyelocytes in the control group, and 8.9 +/- 5.6% in the induction group. Therefore, we think that there is a close correlation between PKC and the differentiation of promyelocytic leukemia cells.