A liposome preparation which contains a water-soluble 2'-deoxycytidine compound encapsulated therein and charges positively the surface of a lipid membrane. As the preparation is expected not only to reduce the toxicity against myeloid tissues of the above compound having a malignant tumor resistance but also to permit accumulation of the active ingredient in the tumor tissue by virtue of an improved capability of residence in the blood, it can retain or improve the antitumor effect without causing any side effect and is useful for treating malignant tumors.
