Six monoclonal antibodies raised against the human placental alkaline phosphatase (ALP) recognizing distinct antigenic determinants on the surface of this isozyme were used for immunohistochemical studies of adult and fetal human testes and testicular germ-cell tumors. ALP reacting with all six antibodies was defined as placental, whereas ALP reacting with some but not all antibodies was labeled as placental-like. ALP reacting with one of the monoclonal antibodies that recognizes a determinant common to intestinal and placental ALP was tentatively considered probably intestinal, unless it reacted with any other monoclonal placental specific antibody. Using this approach, the authors have identified placental ALP in 4 of 7 seminomas, 3 of 7 tumors composed in part or fully of embryonal carcinoma, and 1 yolk sac carcinoma. Placental-like ALP was identified in 2 additional seminomas and 4 embryonal carcinoma-containing tumors, whereas 1 seminoma and 1 benign teratoma were devoid of either placental or placental-like ALP. Trophoblastic giant cells in 2 seminomas and 3 teratocarcinomas expressed only the antigenic determinant common to placental and intestinal ALP. The authors thus show that testicular tumor cells may express either placental or placental-like ALP and that in some instances, the tumor isozyme is antigenically different from ALP found on either fetal or adult testicular germ cells.