The N4-alkylcytosine arabinoside derivative N4-octadecyl-AraC (AraC-Ocd, NOAC) and the (1-octadecylglycero-3-phospho)-AraC (Ocd-GroP-AraC, OPA) conjugate are new lipophilic derivatives of the cytostatic drug 1-beta-D-arabinofuranosylcytosine (AraC) that produce high antileukemic effects in the L1210 murine leukemia model when administered orally or parenterally as liposomal formulations. Between 83% and 100% of the treated animals were cured after five consecutive daily oral drug applications with a total dose of 1 mmol/kg AraC-Ocd or Ocd-GroP-AraC. Corresponding results were obtained after parenteral therapy on days 2 and 6 after tumor inoculation with five- to ten-fold lower concentrations of these two compounds. A comparable cytotoxic activity was found with the orally active AraC-5'-(n-stearyl phosphate). However, because of its strong hemolytic toxicity this derivative cannot be used for parenteral therapy. Another AraC conjugate, which was modified with two long-chain hydrocarbons, the (1-octadecylglycero-3-phospho)-N4-hexadecyl-AraC was, probably because of poor oral bioavailability, only active when applied parenterally. The new lipophilic AraC derivatives AraC-Ocd and Ocd-GroP-AraC are compounds with a high potential for the oral treatment of leukemias and possibly also of solid tumors.