PS (phosphatidylserine) in mammalian cells is synthesized by two distinct base-exchange enzymes, PSS1 (PS synthase 1) and PSS2, which are responsible for the conversion of PC (phosphatidylcholine) and PE (phosphatidylethanolamine) respectively into PS in intact cells. The PS synthesis in cultured mammalian cells is inhibited by exogenous PS, and this feedback control occurs through inhibition of PSSs by PS. In the present study, we purified epitope-tagged forms of human PSS1 and PSS2. The purified PSS2 was shown to catalyse the conversion of PE, but not PC, into PS, this being consistent with the substrate specificity observed in intact cells. On the other hand, the purified PSS1 was shown to catalyse the conversion of both PC and PE into PS, although PSS1 in intact cells had been shown not to contribute to the conversion of PE into PS to a significant extent. Furthermore, we found that the purified PSS2, but not the purified PSS1, was inhibited on the addition of PS to the enzyme assay mixture, raising the possibility that there was some difference between the mechanisms of the inhibitory actions of PS towards PSS1 and PSS2.