Egg phosphatidyl ethanolamine is degraded to lysophosphatidyl ethanolamine and fatty acids by a phospholipase in post-heparin plasma at rates of 10 to 30 μmoles/hr per ml plasma. The optimal system for measuring this enzyme contained albumin and (NH4)2SO4, and the optimal pH was 8.8–9.1. Only slight degradation of egg or beef lecithin occurred.Post-heparin and pancreatic phospholipase differ in heat stability; in the inhibiting effects of EDTA, HgCl2, and diethyl-p-nitrophenyl phosphate; in the requirement for deoxycholate or albumin; and in an apparent specificity of the post-heparin enzyme for the ethanolamine phosphatide.