Transcripts for the muscle regulatory gene MyoD1 are expressed during normal skeletal muscle myogenesis and in rhabdomyosarcomas but not in other tissues or in soft-tissue sarcomas. Here we report the distribution of MyoD1 protein, determined by reactivity with anti-MyoD1 polyclonal sera in normal tissues, rhabdomyosarcoma cell lines, and in a variety of pediatric solid tumors. The distribution of MyoD1 protein was highly restricted in normal tissues and was detected only in fetal skeletal muscle and more faintly in adult skeletal muscle. All six human rhabdomyosarcoma cell lines analyzed expressed MyoD1 mRNA transcripts as well as immunoreactive protein. The immunohistochemical expression of MyoD1 protein was then examined in 49 surgical specimens from a variety of pediatric solid tumors. Each of 16 rhabdomyosarcoma specimens was positive for MyoD1, including four that did not express the intermediate filament protein desmin. Two of five specimens originally designated sarcoma type indeterminate (STI) and two of three specimens originally designated extraosseous Ewing's sarcoma (EOE) were positive for MyoD1, suggesting commitment to myogenic differentiation. Three of eight Wilms' tumors, which also expressed desmin and had clearly evident myogenic elements, also were positive for MyoD1. Tumors that failed to express MyoD1 protein included neuroblastoma, primitive neuroectodermal tumor, non-Hodgkins lymphoma, embryonal sarcoma of the liver, malignant fibrous histiocytoma, malignant rhabdoid tumor, and Ewing's sarcoma of the bone. These results indicate that expression of MyoD1 protein is highly restricted in normal human tissues and that expression of this gene product in malignant tissue may be diagnostic for rhabdomyosarcoma. Furthermore MyoD1 staining may be a valuable adjunct in the classification of pediatric soft-tissue sarcomas.