The activity of phospholipase A2 from cobra venom toward phospholipid in single-walled, sonicated vesicles was analyzed, particularly with respect to its activity toward the saturated phosphatidylcholines in the gel and liquid crystalline states. When egg phosphatidylcholine vesicles are used as substrate, the phospholipase has an apparent Km of 4.4 mM, an apparent Vmax of 100 mumol min-1 mg-1 of protein, and a pH optimum of 5.0 at 40 degrees C. The phospholipase hydrolyzed the gel state of dimyristoyl phosphatidylcholine vesicles and dipalmitoyl phosphatidylcholine vesicles at a rate 2 to 3 times greater than the liquid crystalline state, taking into account temperature effects on the enzymatic reaction itself. The results suggest that, toward sonicated vesicles, there is no specific enhancement of the rate when the both liquid crystalline and gel states are present together, as has been suggested to occur for multibilayers studied with other phospholipases. An apparent stimulation of activity as the reaction proceeded was observed above the phase transition temperature. This might be attributed to an increase in the phase transition temperature caused by free fatty acids so that, in the presence of reaction products, the enzyme is actually hydrolyzing gel state phospholipid which was found to be the preferred lipid state for phospholipase activity.