We observed adiamycin (ADR) uptake and cellular transmembrane potential [amount of intracellular fluorescence of 3,3'- (Di-n-hexyl)- 2,2'- oxacarbocyanine iodide (NK-2280)] in ADR-resistant cells established from Ehrlich ascites tumor cells (EATC) and wild type EATC. In ADR-resistant cells, ADR uptake and the cellular transmembrane potential decreased as the degree of resistance increased. 4,4'- diisothiocyanatostilbene- 2,2'- disulfonic acid (DIDS) induced markedly decreases of ADR uptake and the cellular transmembrane potential. A good correlation was observed between ADR uptake and transmembrane potential in cultured cells