Encapsulation in liposomes of the antitumoral drug 2-methyl 9-hydroxyellipticinium and the consequences of its cytotoxicity in vitro on L1210 leukemia cells and on its antitumoral activity in vivo on leukemic mice inoculated with L1210 cells are described. Provided the drugs is dissolved in the buffer below its critical micelle concentration (10(-4) M), it can be encapsulated in lipid vesicles with a very good yield in the form of a very stable combination with the lipids. The in vitro experiments show that 2-CH3 9-OH-ellipticinium is less cytotoxic against L1210 cells when entrapped than when free in solution. The in vivo experiments on tumor-bearing mice show that encapsulation of the drug reduces its toxicity. Encapsulation maintains the antitumoral activity of the drug or increases it if the leukemia is delayed (10(4) cells injected per mouse instead of 10(5) cells per mouse).