Several studies have reported that cyclosporin A (CsA) causes elevated serum bilirubin and bile acid levels, suggesting hepatobiliary dysfunction. This study examines the effects of CsA on the uptake of radiolabelled bilirubin by rat and human hepatocyte suspensions. The initial uptake rates of [(3)H]bilirubin, over a concentration range of 0.05 to 2.0 mum, were determined in isolated hepatocytes that were preincubated with or without CsA. A saturable process for uptake of bilirubin was observed and kinetic constants calculated. Isolated human hepatocytes showed bilirubin uptake rates similar to those of rodent cells. CsA inhibited bilirubin uptake by both rat and human isolated hepatocytes. The apparent inhibition constants for the effect of CsA on bilirubin uptake were 39 and 26 mum, for rat and human hepatocytes, respectively. This inhibitory effect was evident at a dose of 1 mum, which is similar to CsA blood levels seen in those cases in which hyperbilirubinaemia has been reported. However, hepatocytes isolated from rats administered CsA (10 mg/kg/day ip, in Cremophore, for 5 days) exhibited uptake rates similar to those of cells isolated from control animals treated with only the Cremophore vehicle. Overall, the data are consistent with a receptor-mediated uptake of bilirubin by hepatocytes from rats and humans. The inhibition of bilirubin uptake by CsA may explain, at least in part, the hyperbilirubinaemia observed clinically.