The biological characteristics of a new monoclonal antibody (TO73) reacting with a vincristine-resistant human leukemic cell line (KY-VCR) were evaluated. Immunological and electron-immunological studies showed that TO73 reacted with the surface glycoprotein of KY-VCR. TO73 was found to have no effect on cell growth and intracellular uptake of vincristine. In human neoplastic cell lines, TO73 was found to react with 11 of 27 (41%) cell lines. With regard to de novo primary tumor with one exception, TO73 did not react with any of the examined primary tumor cells. The patient with TO73-positive leukemia died of induction failure due to drug resistance. Complete remission was achieved in the other leukemic patients. These results indicate that TO73 antigen may be associated with immortalization of tumor cells and poor prognosis in some cases.