The phenomenon of multidrug resistance (MDR), that involves the efflux pump P-glycoprotein, can be reversed by a number of substances known as MDR modulators or reversing agents. In the present study we investigated the action of three anthracyclines, mitoxantrone and vincristine on short-term (72 h) cultures using 2 methods ([3H] incorporation and MTT (3-[4,5-dimethylthiasol-2-yl]-2,5-diphenyltetrazolium bromide)), on 2 cell lines: K562, a human erythroleukemia, and a vincristine-resistant subline K562-Lucena 1. Using the same culture methods plus flow cytometry analysis, the reversing potentials of cyclosporin A and verapamil were studied in both cell lines. There were differences in the sensitivity and resistance profiles of the two lines to the various drugs but daunorubicin (5 micrograms/ml) and idarubicin (0.035 micrograms/ml) were the most effective when each was used in high concentration. Cyclosporine at 200 ng/ml and verapamil at 5 micrograms/ml reversed MDR in the resistant line, and had a synergistic action with chemotherapeutic agents on the sensitive line. Again differences were demonstrable between combinations of the various drugs and reversal was only clearly shown with the method measuring cell proliferation ([3H] incorporation) but not by the method measuring metabolic activity (MTT). The efflux of rhodamine-123 mimics the functional activity of the pump and cyclosporine was a better reversing agent by this criteria. These data show that the results obtained in in vitro studies attempting to identify treatments for different types of leukemias depend to a large extent on the methods used to measure cell response.