It has been proposed that P-glycoprotein, the product of the human MDR1 gene, may function not only as a drug transporter but, depending on the conditions, as a volume-activated Cl- channel [Valverde, Diaz, Sepúlveda, Gill, Hyde and Higgins (1992) Nature (London) 355, 830-833; Gill, Hyde, Higgins, Valverde, Mintenig and Sepúlveda (1992) Cell 71, 23-32]. To verify this hypothesis, we have compared volume-activated Cl- currents with the level of MDR1 mRNA and its protein product in the human KB3 (epitheloid lung cancer) and HeLa cell lines. The related MDR2 was also included to find out whether it could account for observed discrepancies between Cl- current and MDR1 expression. A 40% decrease in osmolarity evoked a Cl- current in both cell types (at +80 mV: 50.3 +/- 4.3 pA/pF in KB3, n = 13; 28.2 +/- 3.3 pA/pF in HeLa, n = 16). The blocking of this current in both cell types by 5-nitro-2-(3-phenylpropylamino)-benzoic acid and by 1,9-dideoxyforskolin is similar to that of the presumed P-glycoprotein associated Cl- channel. As measured by reverse-transcriptase polymerase chain reaction, KB3 cells expressed only an extremely small amount of the messengers for MDR1 and MDR2. The signal observed for MDR1 in HeLa cells was at least an order of magnitude more intense than in KB3 cells, while MDR2 mRNA was undetectable. A clear difference in MDR1 expression between KB3 and HeLa was also observed at the protein level. These data are difficult to reconcile with the hypothesis that in HeLa and KB3 cells MDR1- or MDR2- encoded P-glycoproteins are associated with volume-activated Cl- channels.