Protein kinase C (PKC) is a multifunctional protein-Ser/Thr kinase and is generally accepted to be involved in a wide variety of cellular signal transduction. Recent biochemical fractionation of PKC as well as sequence analysis of its cDNA clone has revealed the existence of multiple subspecies of this enzyme with obvious tissue- and cell-specific expression. The present study is concerned with the differential regulation of expression of PKC subspecies of human promyelocytic leukemia cell line HL-60 during differentiation toward granulocyte-like cells induced by retinoic acid. PKC in HL-60 cells is resolved into three distinct subspecies (peak a, b, and c) by the hydroxyapatite column chromatography. Biochemical and immunochemical analyses revealed that peak a and c enzymes were identified with the type II (beta II) and type III (alpha) PKC subspecies previously identified and characterized from brain. On the other hand, peak b enzyme was separable from type I, II and III PKC, and is suggested to be unidentified PKC subspecies possessing subtly different characters from others. Treatment of HL-60 cells with 1 microM retinoic acid caused the decrease in peak b activity within 24 h, while peak a activity was increased and peak c activity was slightly decreased within 48 h. The elution profile of PKC subspecies in human peripheral neutrophils resembled that obtained with HL-60 cells treated with RA. The results imply that activities of PKC subspecies in HL-60 cells may be distinctly regulated by retinoic acid treatment prior to the achievement of cell differentiation.