An organospecific alpha2-globulin of human placenta was identified with the aid of immunochemical analysis. This antigen differed immunologically from the alpha2- and beta1-globulins of pregnancy, alpha-fetoprotein, placental lactogen, and chorionic gonadotropin. High level of the antigen revealed was found in the placental tissue and the amniotic fluid at the early terms of pregnancy, but its concentration sharply decreased by the time of labour.