Adriamycin (ADR) is frequently used in the clinical treatment of cancer. ADR resistance is one of the most serious problems in cancer chemotherapy. Several factors have been demonstrated to be related to ADR resistance. Among these factors, the influx of ADR into cells and the efflux of ADR out of cells are important aspects of its effectiveness. In this study, we investigated the influence of FK506 on the ADR accumulation and retention in the ADR-resistant strain of Ehrlich tumor cells. The ADR accumulation in the ADR--resistant strain was about 40% of that of wild EAT cells. The addition of 50 muM FK506 inhibited the efflux of ADR completely. ADR accumulation with the addition of 50 muM FK-506 to the ADR-resistant strain was about 80% of that found in the wild strain. Thus, it was suggested that the increase in ADR accumulation by the ADR resistant strain by about 40%, when 50 muM FK-506 was added might be due to complete inhibition of ADR efflux by P-glycoprotein, that the decrease of about 20% in ADR accumulation of the ADR-resistant strain compared to that in parent cells might be caused by problems with ADR influx. Therefore, it is necessary that the mechanisms which cause the decrease of ADR influx in the ADR -resistant strain be clarified.