A new species of protein kinase (protein kinase C), found in human peripheral lymphocytes, is 20 times more active than adenosine 3',5'-monophosphate-dependent protein kinase (protein kinase A) with histone as phosphate acceptor. This enzyme is normally present as an inactive form in soluble fraction, but attaches to membrane to exhibit full catalytic activity. This activation process is reversible, and absolutely requires Ca++. No other divalent cations can substitute for Ca++. The enzyme is independent of cyclic nucleotides. The active factor associated with membrane is phospholipid. The protein kinase shows a m.w. of 7.7 X 10(4). The pH optimum is 7.5 to 8.5. GTP does not serve as phosphate donor. The enzyme appears to show relatively broad substrate specificity that is distinctly different from that of protein kinase A. Although both protein kinases C and A react with H1 histone, analysis of the N-bromosuccinimide-bisected fragments of this radioactive histone has revealed that protein kinase C greatly favors seryl and threonyl residues of the C-terminal portion, whereas protein kinase A reacts preferentially with seryl residues in the N-terminal portion of this histone. Protein kinase C is protentially multifunctional and may regulate various Ca++-dependent cellular processes. It is noted that protein kinase C may be alternatively activated in an irreversible manner by limited proteolysis with Ca++-dependent neutral protease. The enzyme activated in this way is independent of Ca++ and membrane.