Ethyl 6-p-5-(l-imidazolyl) pentyloxyphenoxy-2, 2-dimethylhexanoate hydrochloride (YM534) is a new synthetic anti-tumor compound. Combinations of YM534 with other anti-cancer agents were examined to ascertain whether YM534 potentiated other anti-cancer agents against the KB cell line and its multidrug-resistant counterpart, VJ-300. YM534 potentiated the cytotoxic action of vincristine and actinomycin D about 2-fold against KB cells, but not those of daunomycin and adriamycin. By contrast, YM534 only slightly reversed drug-resistance to adriamycin and daunomycin in VJ-300 while it reversed 5-fold vincristine resistance and 60-fold actinomycin D resistance in VJ-300. The reversal effect of YM534 on actinomycin D and vincristine-resistance in VJ-300 cells appeared to be due to enhanced accumulation of [3H] actinomycin D and [3H] vincristine in VJ-300 cells by YM534. YM534 inhibited efflux of actinomycin D and vincristine from VJ-300 cells, and it also enhanced cellular uptake of these anti-cancer agents. YM534 enhanced cellular accumulation of both actinomycin D and vincristine in the sensitive KB cells. YM534 is thus a unique anti-cancer agent since combinations of other anti-cancer agents with YM534 are expected to augment anti-tumor activity of them. By contrast, YM212, a carboxy analog of YM534, had much less activity to potentiate vincristine and actinomycin D). YM534 at 100-1000 microM almost completely inhibited the photoaffinity labeling of [3H] azidopine to the 170-kD P-glycoprotein of VJ-300 cell membranes, but YM212 showed much less inhibitory action on the photoaffinity labeling. YM534 could also inhibit the photoaffinity labeling of deglycosylated P-glycoprotein.