Protein kinase C represents a structurally homologous group of proteins similar in size, structure and mechanism of activation. They can modulate the biological function of proteins in a rapid and reversible manner. Protein kinase C participates in one of the major signal transduction systems triggered by the external stimulation of cells by various ligands including hormones, neurotransmitters and growth factors. Hydrolysis of membrane inositol phospholipids by phospholipase C or of phosphatidylcholine, generates sn-l,2-diacylglycerol, considered the physiological activator of this kinase. Other agents, such as arachidonic acid, participate in the activation of some of these proteins. Activation of protein kinase C by phorbol esters and related compounds is not physiological and may be responsible, at least in part, for their tumor-promoting activity. The cellular localization of the different calcium-activated protein kinases, their substrate and activator specificity are dissimilar and thus their role in signal transduction is unlike. A better understanding of the exact cellular function of the different protein kinase C isoenzymes requires the identification and characterization of their physiological substrates.The eukaryotic cell is a highly regulated entity, responding to its immediate intracellular environment as well as to external stimuli. Most regulations are mediated, either directly or indirectly, by conformational changes in proteins, and the equilibrium between active and inactive conformational states can be altered by both allosteric and covalent mechanisms. A most common covalent means of regulating protein activity is protein phosphorylation, which is particularly prominent for the role that it serves in signal transduction and is important in many other cellular responses (Edelman et al., 1987). Signals impinging on cells have their effects amplified and distributed by a network of protein phosphorylation and dephosphorylation reactions.