L1210 leukemia was used to compare the antitumor activities of three nitrosoureas (chlorozotocin, RFCNU and CNCC) encapsulated in liposomes with those in the free state. The results obtained varied according to the chemical structure of the compound, its solubility in oil or water, the route of administration into the body, and the treatment dose. The application of liposomes in the chemotherapy of malignant disease deserves further investigations.