Protein kinase C functions as the transducer of a second messenger, diacylglycerol, and is the major receptor for tumour‐promoting phorbol esters. The enzyme is a family of proteins with closely but distinct structures and individual enzymological properties. Members of the family are differently distributed in particular cell types and limited intracellular locations from lower organisms to mammalian tissues. The enzyme appears to interact with many signalling pathways, and display functions in the processing and modulation of cellular responses to external stimuli. Presumably, each member of the family plays discrete roles in the control of a variety of membrane functions and activation of gene transcription.