In performing cancer chemotherapy, it is essential to know the expression of multidrug resistant (MDR) P glycoprotein (p-gp) on cancer cells. In the present study, in order to clarify the relationship between MDR of leukemic cells and cytologic, immunological and clinical features of acute nonlymphocytic leukemia (ANLL), leukemic cells in peripheral blood and/or bone marrows obtained from 28 ANLL patients were examined. Each smear was stained with C219 monoclonal antibody against P-gp by the APAAP method, and then 1,000 ANLL cells in each smear were observed. Among the FAB subtypes, M4 showed the highest proportion of leukemic cells expressing P-gp. Concerning the response to chemotherapy, five of seven patients (71%) having 1.0% or more of P-gp positive leukemic cells and 11 of 19 patients (58%) having less than 1.0% of those cells achieved complete remission. However, there was no significant correlation between P-gp expression and clinical outcome. There was also no significant correlation between P-gp expression and CD7 or CD34. Furthermore, no significant correlation between chromosome 7 abnormality and P-gp expression was observed. From these results, if we can clarify the mechanism of MDR and the relationship between MDR and cytogenetic or clinical features of ANLL with further study, P-gp expression may become a useful marker for predicting the outcome of ANLL.