Leukemia cells isolated from eight patients with acute leukemia before treatment were examined for in vitro uptake of daunorubicin (DNR) and inhibition of DNA synthesis. In addition, plasma and cellular levels of DNR and daunorubicinol (DOL) were examined in six of the eight patients. Inhibition of DNA synthesis was determined with a 3H-thymidine incorporation assay. In vitro cellular 14C-DNR was quantified by means of liquid scintillation spectrometry, whereas in vivo DNR and DOL concentrations were determined by high-performance liquid chromatography. In vitro intracellular plateau concentrations of DNR were achieved within 1-2 h after continuous exposure to 0.01, 0.1, and 1.0 microgram/ml in the majority of cases. Based on our in vitro studies, a dose-response curve was found between increasing intracellular DNR and incorporation of 3H-thymidine. Peak intracellular levels of DNR after treatment occurred immediately after administration of the drug, whereas intracellular DOL levels accumulated over several hours. Plasma concentrations of DNR and DOL were not useful in estimating target tissue concentrations or inhibition of 3H-thymidine incorporation. Extrapolation of in vivo cellular DNR concentrations to the in vitro dose-response curve allows an estimate of DNR sensitivity.