Two murine monoclonal antibodies, 29-13 (IgG1) and 29-2 (IgG2a), generated against malignant fibrous histiocytoma plasma membranes immunoprecipitated a Mr 200,000 protein (p200), with an isoelectric point between 6.3 and 7.5. Two additional antibodies, 35-16 (IgG1) and 30-40 (IgG2a), generated against Ewing's sarcoma membranes, immunoprecipitated an acidic protein of Mr 160,000 (p160), with an isoelectric point between 5.8 and 6.7. Monoclonal antibodies 29-13 and 29-2 recognize a similar determinant(s) on p200 while 35-16 and 30-40 recognize different determinants on p160. Monoclonal antibody 29-13 exhibited significant binding to membranes isolated from fibrosarcoma and aggressive fibromatosis; moderate binding to osteosarcoma, hemangiopericytoma, and malignant fibrous histiocytoma; and minimal to no binding to other soft tissue sarcoma plasma membranes. The p200 protein was not expressed in 16 other malignant tumors and in only 3 of 35 normal human tissue specimens. High levels of p200 were selectively expressed by leiomyosarcoma, Ewing's sarcoma, and fibrosarcoma cells as well as neonatal fibroblasts in vitro, but not by other carcinoma cell lines or B-lymphoblasts. The p160 protein appeared to be selectively expressed by Ewing's sarcoma with little or no expression on other sarcomas, carcinomas, or normal tissues. However, the p160 antigen was expressed in Ewing's sarcoma, leiomyosarcoma, melanoma, 4 of 9 carcinomas, and neonatal fibroblasts in vitro. The affinity of MoAbs 29-13, 29-2, 35-16, and 30-40 ranged from 5.3 x 10(8) to 4.7 x 10(9) M-1 for sarcoma membranes with approximately 5 x 10(4) binding sites/sarcoma cell.