Using a sonicated dispersion of radiolabeled 1-palmitoyl-2-arachidonoyl- sn-glycero-3-phosphocholine as substrate, we found that phospholipase A 2 activity of human platelets was enhanced 2.4-fold by albumin (1 mg/ml). The enzyme was recovered predominantly in the cytosolic fraction of platelets with less than a third of its activity being associated with the membrane fraction. In the presence of 24 mM n- octyl-β- d- glucopyranoside (octylglucoside) phospholipase A 2 was effectively (more than 90%) extracted from platelet lysates without solubilization of platelet membranes. Ion exchange chromatography of the soluble enzyme yielded a phospholipase A 2 of unchanged total activity and great stability. This phospholipase A 2 was active only in the presence of divalent cations (Ca 2+ > Sr 2+ > Mg 2+ = 0), required albumin for optimal activity and exhibited exclusive positional specificity for the acyl ester bond at the 2-position of 1-palmitoyl-2-arachidonoyl- sn-grycero-3-phosphocholine. Indomethacine (500 μM), mepacrine (500 μM) and N-ethylmaleimide (4 mM) inhibited the phospholipase A 2 by 69, 62 and 19%, respectively. The results are discussed in the light of previous findings on human platelet phospholipase A 2.