Both the tissue lactate dehydrogenase (LDH) isoenzyme pattern and total LDH activity were investigated in a series of human uterine sarcomas and cell lines derived from these tumors. Abnormal LDH isoenzyme patterns characterized by shifts toward the muscle isoenzyme forms (LDH4 and 5) were found in some, but not all, of the sarcomas. The tumors containing rhabdomyosarcomatous elements usually had normal or only slightly altered LDH isoenzyme patterns. The LDH isoenzyme patterns expressed by cultured cell lines established from the uterine sarcomas, in general, resembled those found in the primary tumors. These results suggest that the normal or near normal LDH isoenzyme distributions found in uterine sarcomas with rhabdomyosarcomatous differentiation may be a potential characteristic which may be used for distinguishing these tumors from other uterine sarcomas.