A tumor-promoting phorbol ester, [3H]phorbol-12,13-dibutyrate, may bind to a homogeneous preparation of Ca2+-activated, phospholipid-dependent protein kinase (protein kinase C) in the simultaneous presence of Ca2+ and phospholipid. This tumor promoter does not bind simply to phospholipid nor to the enzyme per se irrespective of the presence and absence of Ca2+. All four components mentioned above appear to be bound together, and the quaternary complex thus produced is enzymatically fully active for protein phosphorylation. Phosphatidylserine is most effective. Various other phorbol derivatives which are active in tumor promotion compete with [3H]phorbol-12,13-dibutyrate for the binding, and an apparent dissociation binding constant of the tumor promoter is 8 nM. This value is identical with the activation constant for protein kinase C and remarkably similar to the dissociation binding constant that is described for intact cell surface receptors. The binding of the phorbol ester is prevented specifically by the addition of diacylglycerol, which serves as activator of protein kinase C under physiological conditions. Scatchard analysis suggests that one molecule of the tumor promoter may bind to every molecule of protein kinase C in the presence of Ca2+ and excess phospholipid. It is suggestive that protein kinase C is a phorbol ester-receptive protein, and the results presented seem to provide clues for clarifying the mechanism of tumor promotion.