FIELD : medicine.SUBSTANCE : invention relates to medicine, in particular to an antitumor composition, which is an aqueous solution of doxorubicin and an inhibitor of ATP-dependent reverse cell transporters in a 1 : 15 mass ratio, where the inhibitor of ATP-dependent reverse cell transporters is a mixture of polyoxypropylenehexol of formula 4 , where n=2–6, and polyoxypropylene glycol of the formula 5 , where m=5–9, having a hydroxyl number of 215–240 mg KOH/g, compound 4 has a molecular weight from 1, 000 to 1, 500 Da, compound 5 has a molecular weight from 300 to 500 Da, the molar ratio of compounds 4 and 5 is 0.9–1.1.EFFECT : invention provides an increase in the efficacy and safety of the action of the antitumor drug doxorubicin, even for the most aggressive types of multi-drug resistant malignant tumors.1 cl, 16 dwg, 3 ex