Phosphatidylserine from human plasma was purified and identified by amino normal-phase and octadecylsilyl reversed-phase high-performance liquid chromatography, respectively. The two major molecular species within the human plasma phosphatidylserine, qualitatively determined by negative-ion fast atom bombardment mass spectrometry, are 1-stearoyl-2-oleoyl-sn-glycero-3-phosphoserine and 1-stearoyl-2-arachidonyl-sn-glycero-3-phosphoserine.