A high-performance liquid chromatographic method for the quantitative simultaneous measurement of daunorubicin and its active metabolite, daunorubicinol, at low concentrations in human plasma was described. The advantages of this method were specificity, sensitivity, minimal pre-analysis work-up and small sample size. The method was sensitive for plasma levels of daunorubicin and daunorubicinol as low as 10 ng/ml. Preliminary dosages on plasma from leukemic patients treated with a 4 hr infusion of 75 mg/m2 daunorubicin either as free drug or complexed with DNA were reported. The results suggested a slower disposition of the daunorubicin-DNA complex.