Doxorubicin serum levels were measured with a highly specific reversed-phase HPLC-method in patients undergoing combination chemotherapy. Serum elimination kinetics of doxorubicin follow a bi- or triphasic first order pattern with a high interpatient variability. T 1/2 of the first distribution phase was 5.4 min. T 1/2 of the terminal steady-state elimination phase was 1273 min. After consecutive bolus application of 15 mg/m2 (4 times per 48 h) cumulation of doxorubicin was found in patient's serum. Serum levels 4 h after bolus injection of doxorubicin were found to be correlated with the area under the elimination curve. Therefore, 4 h serum levels after bolus injection can be used for clinical studies correlating pharmacokinetics with antineoplastic properties of doxorubicin.