N4-Hexadecyl-1-beta-D-arabinofuranosylcytosine (hxd4araC), a new cytostatic derivative of the antileukemic drug 1-beta-D-arabinofuranosylcytosine (araC), was linked in gram-scale syntheses to phospholipids containing differently substituted glycerol residues. All phospholipid-araC conjugates which were condensed via a phosphotriester linkage were shown to be ineffective in the in vivo treatment of L1210 murine leukemia. The transformation of the triesters into phosphodiester-linked conjugates by cleavage of the 2-chlorophenyl protecting group resulted in conjugates which were highly active against L1210 leukemia. These conjugates form stable liposomes with matrix lipids which exert antileukemic effects depending on the number and characteristics of the lipophilic residues of the conjugates. By treatment of L1210 leukemic mice with 100 mumol/kg body wt as total dose given by i.p. injection on days 2 and 6 after tumor inoculation with liposomal hxd4araC or 1-O-octadecyl-rac-glycero-3-phosphoryl-(3-->5')-1- beta-D-arabino-furanosylcytosine (Ocd1GroP-araC) the fraction of 60-day survivors was 100%. Corresponding curative effects were observed after treatment with 200 mumol/kg of 1,2-O-dipalmitoyl-rac-glycero-3-phosphoryl-(3-->5')-N4-palmitoyl-1-beta- D-arabinofuranosylcytosine (Pam1pam2GroP-pam4araC); 1,2-O-dioctadecyl-rac-glycero-3-phosphoryl- (3-->5')-N4-palmitoyl-1-beta-D-arabinofuranosylcytosine (Ocd1ocd2GroP-pam4araC) or 1-O-octadecyl-rac-glycero-3-phosphoryl-(3-->5')-hxd4araC (Ocd1GroP-hxd4araC). Four other conjugates with differently combined palmitoyl-, octadecyl- and hexadecyl residues were significantly less active or inactive. A distinct relationship between the chemical structures and the antileukemic activity of the nine investigated compounds was not found.