We investigated the possibility of liposomes being a drug carrier for cis-diamminedichloroplatinum (II) (CDDP). Liposomes were prepared by sonic dispersion of egg phosphatidylcholine: cholesterol (13: 5 molar ratio) in an aqueous solution containing CDDP. The leakage of entrapped CDDP was examined by means of atomic absorption spectrometry. The release of CDDP from liposomes in RPMI 1640 supplemented with 20% fetal bovine serum at 37°C was rapid and depended on the incubation time. The amount released in 48 hours was about 70%. However, liposomes were stable in PBS (-) at 4°C (about 90% latency after 48 hours). Compared with the activity of free CDDP, CDDP encapsulated in liposomes showed decreased antitumor activity in vitro for a human malignant melanoma cell line and two human osteosarcoma cell lines. These results suggest that the antitumor activity of entrapped CDDP was only due to its release from liposomes bound to cells.