Hybridoma cells were derived from a mouse immunized with plasma membranes prepared from the fresh tumor tissues of a patient with malignant fibrous histiocytoma (MFH), a soft tissue sarcoma. Supernatants from the resultant hybridoma clones were screened for positive antibody binding to tumor membranes and negative binding to membrane preparations of normal tissues using a solid-phase radioimmunoassay. Two distinct monoclonal IgG1 (kappa) antibodies, 19-14 and 19-24, were identified that showed identical patterns of reactivity with a large panel of tissues. Both antibodies displayed high levels of binding to membranes prepared from a majority of MFH and osteogenic sarcoma tumors tested. Moderate levels of binding were obtained with melanoma, colorectal carcinoma, and first-trimester fetal membranes. Weak or no significant binding was observed with membranes from a variety of autologous and allogeneic normal adult tissues. Antibody reactivities could be specifically removed by absorption with MFH and osteosarcoma membranes but not with adult muscle membranes. An electrophoretic analysis of immunoprecipitated membrane antigens indicated that antibodies 19-14 and 19-24 reacted with the same protein, a monomer with an approximate molecular weight of 102,000. The antigen was detected in membrane preparations of MFH, osteosarcoma, and first trimester fetus, but was not present in normal adult spleen. However, a small amount of antigen of molecular weight 107,000 was precipitated from a normal adult liver preparation, which suggests that related antigens may be present in low levels in some normal tissues. Antibodies 19-14 and 19-24 also specifically bound to intact, cultured MFH cells, indicating that the relevant antigens were expressed on the outer cell surface.