Cellular accumulation and intracellular distribution of the anthracycline epirubicin (EPI) were studied by flow cytometry and confocal laser scan microscopy in resistant (HB8065/R) and sensitive (HB8065/S) human hepatoma cells. Using peroxidase immunohistochemistry HB8065/R cells were shown to express the multidrug efflux transporter P-glycoprotein (Pgp). Net drug accumulation was detectable in both cell types within seconds of treatment, and the intracellular drug level increased to a plateau after 15 min in HB8065/R cells and after 90 min to a 4.2 times higher level in HB8065/S cells. A 50% growth inhibition (GI50) was obtained in HB8065/R cells with 46 times as high EPI dose as in HB8065/S cells. Verapamil (VPL) increased the cellular accumulation of EPI and decreased the growth inhibition in HB8065/R cells. The cellular pharmacokinetics and cytotoxicity of EPI in HB8065/R cells reflect the increased levels of Pgp compared to HB8065/S cells.