OATP1B1 (a.k.a. OATP-C, OATP2, LST-1, or SLC21A6) is a liver-specific organic anion uptake transporter and has been shown to be a higher affinity bilirubin uptake transporter than OATP1B3. Using human embryonic kidney (HEK 293) cells stably transfected with OATP1B1, we have studied the effects of indinavir, saquinavir, cyclosporin A, and rifamycin SV on human OATP1B1 transport function. These drugs are potent inhibitors of OATP1B1 transport activity in vitro. We further provide evidence that the calculated fraction of OATP1B1 inhibited at the clinical exposure level correlated very well with the observed hyperbilirubinemia outcome for these drugs in humans. Our data support the hypothesis that inhibition of OATP1B1 is an important mechanism for drug-induced unconjugated hyperbilirubinemia. Inhibition of OATPs may be an important mechanism in drug-drug and drug-endogenous substance interactions.