A new series of highly lipid-soluble cis-bis-carboxylato(trans-R,R-1,2-diaminocyclohexane)platinum(II) complexes were synthesized and characterized by their elemental analysis and by various spectroscopic techniques [infrared (IR), 195pt nuclear magnetic resonance (NMR)]. cis-bis-Neopentanoato(trans-R,R-1,2-diaminocyclohexane)platinum(II ) (NPDP), cis-bis-neodecanoato(trans-R,R-1,2-diaminocyclohexane)-platinum(II ) (NDDP), and cis-bis-n-decanoato(trans-R,R-1,2-diaminocyclohexane)-platinum(II) (DEDP) complexes were entrapped in multilamellar vesicles composed of dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylglycerol (DMPG) at a 7:3 molar ratio and tested for toxicity and antitumor activity. The entrapment efficiency of the liposomal platinum (L-Pt) complexes (L-NPDP, L-NDDP, L-DEDP) was greater than 95%, and the stability in 0.9% NaCl solution at 4 degrees C was greater than 95% at day 14 in each case. The LD50 values of L-NPDP, L-NDDP, and L-DEDP when injected i.v. were 30, 54, and 150 mg/kg, respectively. L-NPDP, L-NDDP, and L-DEDP had no significant nephrotoxicity [as evidenced by a lack of elevated blood urea nitrogen (BUN) levels]. The percentages of T/C obtained after a single i.p. injection of the optimal dose of L-NPDP, L-NDDP, and L-DEDP tested against L1210 leukemia were 175%, 187%, and 212%, respectively [160% for cisplatin (CDDP)]. When a multiple i.p. injection schedule was used (on days 1, 5, and 9), L-NPDP, L-NDDP, and L-DEDP were more active than CDDP (percentage of T/C: 312%, 312%, 277%, and 220%, respectively). When injected i.v., only L-NDDP showed significant activity against L1210 leukemia i.v. (percentage of T/C: 186%). L-NDDP and L-DEDP were markedly active against L1210 leukemia resistant to CDDP (percentage of T/C: 200% and 145% vs 112% for CDDP). L-NPDP, L-NDDP, and L-DEDP also had good activity against i.p. B16 melanoma when they were injected i.p. on days 1, 5, and 9 (percentage of T/C: 206%, 225%, and 306%, respectively). L-NDDP and L-DEDP were more effective than CDDP in inhibiting the growth of liver metastases of murine M5076 reticulosarcoma, whereas L-NPDP was not active. The results obtained to date suggest that L-NDDP is the best L-Pt-complex candidate for further developmental studies.