Human chorionic gonadotropin, the pregnancy hormone, is synthesized by trophoblast cells which make up the placenta. To determine whether antibody to hCG can be used to specifically detect living trophoblast in vitro by binding to the external membrane. Trophoblast was isolated from fresh placentas of women undergoing termination of pregnancy in the first trimester and incubated with monoclonal antibody to hCG. Anti-mouse immunoglobulin G with a fluorescent marker was then added. Syncytiotrophoblast stained positive on the external surface of the cell, while controls of leukocytes, endometrial cells and hepatocytes were negative. The hCG monoclonal antibody may be used to specifically detect hCG on the surface of living trophoblast in vitro.