We previously reported the characteristics of a murine monoclonal antibody (TO73) against the multidrug-resistant (MDR) human leukemic cell line. In the present study, we have studied the clinical significance of TO73 by examining its reactivity with primary and relapsed leukemic cells. Of 20 patients with primary leukemia and 15 patients with relapsed leukemia, none and four cases, respectively, reacted with TO73 (p < 0.05). Furthermore, three of the four positive cases were also positive for P-glycoprotein. These results indicate that TO73 preferentially reacts with leukemic cells resistant to chemotherapy and may be a new marker for refractory leukemia.