1. Saturated and unsaturated phosphatidylcholines, dispersed as liposomes in water, can be hydrolysed by phospholipase A2 from pig pancreas. A pure saturated phosphatidylcholine is hydrolysed only near its transition temperature. An unsaturated phosphatidylcholine is hydrolysed preferentially near its transition temperature, but hydrolysis can occur also above the transition temperature, albeit at a much lower rate. 2. An equimolar mixture of dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine, which shows cocrystallization of the paraffin chains, is hydrolyzed between 25 and 40 degrees C with a maximum at 32 degrees C, in agreement with the calorimetric scan of the phase transition. 3. An equimolar mixture of dilauroyl phosphatidylcholine and distearoyl phosphatidylcholine, which shows a monotectic behaviour, is hydrolysed at all temperatures. Hydrolysis is maximal at 0 and 40 degrees C, at which temperatures dilauroyl phosphatidylcholine and distearoyl phosphatidylcholine undergo their phase transition, respectively. 4. Both in the mixture showing cocrystallization and in the mixture in which phase separation occurs, the phosphatidylcholine species with the shorter fatty acid chains is hydrolysed at a higher rate than the longer chain fatty acid species. 5. Hydrolysis is inhibited by the presence of cholesterol in liposomes prepared of saturated phosphatidylcholine. Inhibition is complete at a cholesterol concentration of 35 mol %. Subsequent addition of filipin and amphotericin B to the mixed cholesterol-phosphatidylcholine liposomes overcomes the inhibitory effect of cholesterol.