The immunohistochemical localization in the human placenta of new placental proteins PP1, PP19, and PP21 was clarified using modified indirect enzyme-labeled antibody method and compared with that of pregnancy-specific β1-protein (SPl). The major results are as follows: positive staining for PP1 was seen at the nucleus and cytoplasm of villous cytotrophoblasts, the X cells at the basal plate, and of chorionic trophoblasts, while the decidua cells and amnion were not stained. PP19 was characteristically seen in the nucleus and cytoplasm of syncytiotrophoblasts. X cells in basal plate, chorionic trophoblasts, and maternal leukocytes. The villous cytotrophoblasts, decidua cells, and amnion were not stained. PP21 localization was found at the microvilli and basal membrane of syncytiotrophoblasts and at the cytotrophoblast plasma membrane of the chorionic villus in early gestation. In late gestation, increased staining was seen at the syncytiotrophoblast microvilli and the villous basement membrane, and moderate staining at plasma membrane of the amniotic epithelium and chorionic trophoblasts. SPl was found only at the syncytiotrophoblast cytoplasm of chorionic villi. Studies using these four placental proteins simultaneously may therefore provide a new key learning about unknown metabolic functions of trophoblasts.