Protein kinase C (PKC) is a family of enzymes that are physiologically activated by 1,2-diacylglycerol (DAG) and other lipids. To date, 11 different isozymes, alpha, betaI, betaII, gamma, delta, epsilon, nu, lambda(iota), mu, theta and zeta, have been identified. On the basis of their structure and activators, they can be divided into three groups, two of which are activated by DAG or its surrogate, phorbol 12-myristate 13-acetate (PMA). PKC isozymes are remarkably different in number and prevalence in different cell lines and tissues. When activated, the isozymes bind to membrane phospholipids or to receptors that are located in and anchor the enzymes in a subcellular compartment. Some PKCs may also be activated in their soluble form. These enzymes phosphorylate serine and threonine residues on protein substrates, perhaps the best known of which are the myristoylated, alanine-rich C kinase substrate and nuclear lamins A, B and C. The enzymes clearly play a role in signal transduction, and, because of the importance of PMA as a tumor promoter, they are thought to affect some aspect of cell cycling. How PKC takes part in the regulation of cell transformation, growth, differentiation, ruffling, vesicle trafficking and gene expression, however, is largely unknown.