Monoclonal antibodies (MoAbs) against human osteosarcoma cells were obtained by the production and cloning of hybrids resulting from the fusion of mouse myeloma cells P3 X 63Ag8.653 with spleen cells from partially purified, osteosarcoma-associated antigen (OSAA)-immunized BALB/c mice. OSAAs were isolated from the spent culture medium of a human osteosarcoma cell line (TE-85). Five hybrid clones were established and designated as OSA1, OSA2, OSA3, OSA4, and OSA5. OSA1 and OSA2 had similar activity. All 5 MoAbs reacted strongly with most osteosarcoma cell lines and with all osteosarcoma tissues tested but not with 10 tumor cell lines and 2 tumor tissues from other cancers. OSA3, OSA4, and OSA5 cross-reacted with a fibrosarcoma cell line, a colon cell line, and fibrosarcoma, respectively, as well as with a melanoma cell line. None of the MoAbs were reactive with activated normal human peripheral blood mononuclear cells (PBMC). Immunoprecipitation of membrane protein isolated from LM cells and TE-85 cells with the MoAbs OSA1 and OSA2 conjugated with Staphylococcus aureus yielded a molecule with molecular weight of approximately 92,000. No detectable membrane protein was precipitated when 125I-labeled membrane protein from pooled activated human PBMC and tumor cells of other histologic types were used in the immunoprecipitation.