In our previous report (Hayakawa, M., Kudo, I., Tomita, M., & Inoue, K. (1988) J. Biochem. 103, 263-266), we have shown that phospholipases A₂, purified from rat platelet membrane fractions and an extracellular medium of thrombin-stimulated rat platelets were essen-tially identical to each other. Both purified enzymes were digested with proteases, and the resulting peptides were subjected to primary sequence determination. The sequence analysis of the HPLC-separated peptides and the alignment of the sequences showed a tentative primary structure of rat platelet phospholipase A₂, which was composed of 125 amino acid residues. It showed 47% homology with snake venom Agkistrodon halys blomhoffli phospholipase A₂.