The effect of temperature and various glycerides to modulate the ability of lysolecithin and fatty acid to promote high phospholipase A2 activity was studied using dipalmitoylphosphatidylcholine large unilamellar vesicles as substrate. The length of the lag phase prior to the accumulation of sufficient hydrolysis products (lysolecithin and fatty acid) to support high phospholipase activity was shortest at temperatures near the thermotropic phase transition of the phospholipid substrate. A reduction in the lag phase correlated with a reduction in the requirement for hydrolysis products at the phase transition temperature, where the bilayer exists in a state of fluctuating domains of gel and liquid crystal. Dipalmitoylglycerol and tripalmitoylglycerol also reduced the length of the lag phase. This reduction was both concentration-dependent and temperature-dependent relative to the phase transition in the presence of the glycerides. As with the effect of temperature, the ability of di- and triglycerides to decrease the lag time correlated with a decrease in the amount of reaction products necessary to promote high phospholipase activity. This effect coincided with the tendency of the glycerides to form domains in the bilayer. Glycerides that did not form domains either had no effect (monopalmitoylglycerol) or increased the length of the lag phase (dicaprylglycerol). These data suggest that the effect of the reaction products to increase phospholipase A2 activity is aided by the presence of fluctuations in lipid domains within the bilayer.