Intracellular phospholipases (A2 and C types) can hydrolyse bilayer-forming phospholipids much more rapidly when diacylglycerol is added. Unsaturated long-chain diacylglycerols are much more effective than short-chain saturated diacylglycerols or 1-oleoyl,2-acetylglycerol. Diacylglycerol does not change the electrokinetic properties of the phospholipid water interface, nor does it enhance enzymic digestion of monolayers. 31P-NMR of phosphatidylcholine indicates that diacylglycerol causes an isotropic component to develop in the spectrum of the bilayers which correlates approximately with the enhancement of phospholipase A2 attack. Addition of further diacylglycerol converts this transitional stage of unknown structural origin to the hexagonal II phase with a total loss of enzyme activity.