The use of sonicated phospholipid vesicles (liposomes) as carriers of 2-[3'-(methoxycarbonylamino)-phenyl]-3-phenyl-6-methoxycarbonylamino-4-(3H)- quinazolone (NSC-251635), a water-insoluble antimitotic compound, was investigated in mice. NSC-251635 was incorporated in egg yolk lecithin, cholesterol, and stearylamine (4:3:1) liposomes. In vitro, NSC-251635 in suspension or entrapped in liposomes was not toxic for L1210 cells. In vivo, after ip or iv injections to CDF1 mice bearing intraperitoneal or intravenous L1210 leukemia, NSC-251635 was active only when it was incorporated in the liposomes and not when it was given as a suspension in Klucel or in saline. The NSC-251635 liposome preparation induced significantly prolonged survival of the treated animals.