INTRODUCTION

In recent years the plasma membrane components of the human trophoblast have attracted increasing interest. This paper describes progress in two directions, the first being to isolate normal human placental cytotropho blast cells in order to characterize their histocompatibility antigens. The second relates to the enzyme placental alkaline phosphatase (PLAP), which is the quantitatively major cell membrane protein of the syncytiotrophoblastic microvilli.