Placental alkaline phosphatase (PLAP) is normally produced by primordial germ cells and syncytiotrophoblasts, and the detection of its expression has been useful in the diagnosis of germ cell tumors. We have recently observed PLAP immunoreactivity in normal human adult and fetal muscle tissue. Based on this observation, we explored the possible role of PLAP in the diagnosis of soft tissue tumors. A total of 271 tumors were studied. These included tumors with myogenic, neural, fibrous, myofibroblastic, lipomatous, neuroepithelial, perivascular, and epithelial differentiation. A formalin-fixed, paraffin-embedded section from each tumor was stained with PLAP monoclonal antibody using standard immunohistochemical methods preceded by antigen retrieval. In addition, western blotting with PLAP monoclonal antibodies was performed on fresh samples from a uterine leiomyoma, grossly normal myometrium, and placenta. Also, formalin-fixed sections of fetal skeletal muscle were labeled with double immunohistochemistry techniques using antibodies to myogenin and PLAP. Cytoplasmic PLAP reactivity was detected in all leiomyomas and rhabdomyosarcomas (100%), 7 of 15 (46%) leiomyosarcomas, 15 of 19 (79%) desmoplastic small round cell tumors, 2 of 15 (13%) gastrointestinal stromal tumors, 1 of 8 (13%) Wilms' tumors, 1 of 9 synovial sarcomas (9%), and 2 of 7 (29%) myofibroblastic tumors. No PLAP reactivity was detected in hyperplastic scars, nodular fasciitis, or the other remaining soft tissue and epithelial tumors. Double immunohistochemistry studies showed coexpression of myogenin and PLAP in fetal skeletal muscle cells, and western blot analysis showed a 70-kDa band in samples derived from grossly normal placenta, benign myometrium, and a uterine leiomyoma. PLAP immunoreactivity is detected in soft tissue tumors with known myogenic differentiation. PLAP immunoreactivity seems to relate to the degree of myogenic differentiation in soft tissue tumors and is more frequently expressed in cells with skeletal muscle differentiation and least in those with myofibroblastic features. The biologic function of PLAP in muscle and tumors with myogenic differentiation is unknown and merits further investigation. In addition to its role as a germ cell marker, PLAP may also be used as a myogenic marker in the diagnosis of soft tissue tumors.