Mouse-human heterohybridoma (3H12) producing human antibody was established by fusing P3/X63-Ag-U1 (P3U1) myeloma cells with lymphocytes from a patient of small cell lung carcinoma (SCLC). This monoclonal antibody reacts to lung cancer cells, especially SCLC, but not to adenocarcinoma or squamous cell carcinoma cells. It does not show any reactivities to other tumors or normal cells so far examined. An immunoprecipitation experiment with this antibody revealed that the antigen on SCLC was a single chain moiety of 150 kilodaltons (Kd). Judging from the cell type reactivity and molecular size of the antigen, this monoclonal antibody appears to detect a new tumor-associated antigen on human SCLC.