The pharmacokinetics of daunorubicin and doxorubicin were studied in plasma and leukemic cells from 16 patients with acute nonlymphoblastic leukemia during 19 courses of treatment with the unconjugated or DNA-conjugated drugs. Daunorubicin and doxorubicin are high-clearance drugs with very high apparent volumes of distribution, indicating a pronounced tissue affinity. This was more pronounced in the case of doxorubicin and may explain the reduced cardiotoxicity of the DNA-complexes. Daunorubicin reached higher intracellular peak concentrations than doxorubicin, but the latter drug was retained much longer. The cell/plasma concentration ratio was higher for daunorubicin than for its reduced metabolite daunorubicinol. No doxorubicinol was found intracellularly. The observed differences in cellular pharmacokinetics between daunorubicin and doxorubicin may explain the difference between the clinical activity spectras of these two drugs. DNA-conjugation did not markedly modify the uptake of daunorubicin in the leukemic cells, whereas the mean intracellular accumulation of doxorubicin was 60% higher when the drug was administered as a DNA-conjugate. This may enhance the selectivity of doxorubicin in the treatment of acute leukemia.