Protein kinase C is synergistically activated by the presence of calcium, certain phospholipids and a diacylglycerol. The physiological activation of the enzyme appears to be determined by the availability of the diacylglycerol which is itself a product of (poly) phosphoinositol turnover. It is shown here that the diacylglycerol activation effect is stereospecific, with only the 1,2-sn-diglycerides being active. This demonstrates for the first time a stereospecific effector role for a membrane-bound lipid. Furthermore, this work strengthens the link forged between the highly potent and specific tumor promoters (such as the phorbol esters) and the diglycerides as activators of protein kinase C.