Forty one patients with acute myeloid leukemia (AML), including 27 at presentation and 14 relapsed or resistant cases, were assessed for laboratory evidence of the MDR phenotype. Leukaemic cells from all 41 cases were studied by immunocytochemistry using the JSB-1 monoclonal antibody and simultaneously by reverse transcription polymerase chain reaction (RT-PCR) to evaluate expression of the mdr 1 gene. Cells from 32/41 cases were also assessed for daunorubicin (DNR) accumulation and retention by flow cytometry (FC). Nineteen of the 41 (46%) patients were positive for MDR by JSB-1 immunocytochemistry (11/27 [41%] at presentation and 8/14 [57%]relapsed or resistant cases). Nine of the 19 (47%) P-gp positive, de novo patients achieved complete remission. 22 patients were negative by JSB-1 immunocytochemistry (16/27 [59%] at presentation and 6/14 [43%]of the relapsed or resistant cases) and 11/22 (50%) P-gp negative patients achieved a complete remission.Of the 32 patients assessed by FC, 7 (22%) were positive for the MDR phenotype with increased DNR accumulation and retention in the presence of the MDR reversing agent verapamil (VPM). 6/7 of the FC positive cases were also JSB-1 positive, and 6 had additional poor risk features. Of the twenty five FC negative patients, 6 had received previous chemotherapy and 15 (60%) achieved complete remission. Mdr 1 mRNA levels were increased in all seven FC positive cases whereas only 7/19 JSB-1 positive cases had raised mdr 1 mRNA levels.These results suggest that the assessment of MDR status by immunocytochemistry using JSB-1 is not predictive of response to chemotherapy. Flow cytometric analysis of blast cells appears to correlate well with mdr 1 mRNA levels and may be a better predictor of treatment outcome.