Partial purification of alkaline phospholipase A2 (EC 3.1.1.4) from rabbit platelets was carried out and the effect of different physical states of the substrate phosphatidylcholine on the activity was investigated. (1) The enzyme was purified about 1020-fold by means of Sephadex gel chromatography after extraction from a particulate fraction of rabbit platelets, followed by CM-cellulose chromatography, and had a molecular weight of approx. 12 000 as determined by gel chromatography. (2) The activity of the purified enzyme was enhanced by the addition of detergents. Sodium deoxycholate and sodium cholate markedly stimulated the activity, and the effect of these substances was observed well below the critical micelle concentrations. Triton X-100 stimulated the activity moderately, and the activation was observed only above the critical micelle concentration. (3) The addition of negatively charged phospholipids to the substrate egg phosphatidylcholine induced a moderate activation of hydrolysis. (4) The addition of long-chain cation to the substrate induced an inhibition of the activity, whereas the addition of long-chain anion activated the hydrolysis of egg phosphatidylcholine, but did not activate the hydrolysis of phosphatidylcholine in the total lipid extract of rabbit platelets. (5) Hydrolysis of dimyristoyl phosphatidylcholine increased in the temperature region of the phase transition of the substrate. Addition of cholesterol at the concentration of 20 mol% diminished the effect of phase transition. (6) Release of [1-14C]arachidonic acid from an equimolar mixture of egg phosphatidylcholine with dipalmitoyl or distearoyl phosphatidylcholine was activated at the temperature of 0 degrees C or 20 degrees C, respectively. From these results, we suggest that platelet phospholipase A2 can be activated to release fatty acids from the platelet phospholipids at the domains within membranes, where exist the structural irregularities and/or accumulation of negative charge within the bilayers.