According to this invention there is provided a new stabilized antitumor composition comprising a mixture of an antitumor anthracycline compound of formula (I)
or a non-toxic salt thereof with lactose, with the lactose being incorporated as a stabilizing agent in the mixture in an amount effective to stabilize the anthracycline compound of formula (I) or its salt from its degradation and thus with the weight ratio between the anthracycline compound or its salt and lactose being in a range of 1 : 0.5 to 1 : 8. There is also provided a method for stabilizing the anthracycline compound of formula (I) or its salt by mixing with lactose in the required proportions.