A noncovalent adduct of the antineoplastic drug cis-diamminedichloroplatinum (cDDP) and a biocompatible graft copolymer of poly(L-lysine) and methylpoly(ethylene glycol) succinate is described. Upon incubation of cDDP with [O-methylpoly(ethylene glycol)-O'-succinyl]-N- epsilon-poly(L-lysine)n-N-epsilon-succinate, n = 250-270, highly soluble, long circulating adducts were formed which contained 4.3% of platinum by weight. Approximately 60% of the polymer-associated drug was released during dialysis against saline or serum albumin containing saline, with a half-time of release of 63 h. The adducts showed a pronounced antineoplastic effect in BT-20 human adenocarcinoma cell cultures. In cell proliferation assays, the concentration of half-inhibition of [3H]thymidine uptake was 0.9 +/- 0.2 microM for the drug-copolymer adduct compared to 0.3 +/- 0.1 microM for free cDDP. The adduct showed a long blood half-life (ca. 14 h in rats) and accumulated in experimental mammary adenocarcinomas at 2.5-3.5% injected dose per gram of tissue. A control adduct of cDDP with the backbone portion of the copolymer, poly(L-lysine)-N-epsilon-succinate, had a short half-life in the bloodstream (ca. 30 min) and low accumulation (0.5% injected dose per gram) in tumor. A dual therapeutical effect of methylpoly(ethylene glycol)succinylpoly(L-lysine)-succinate as a carrier of cDDP is suggested: (1) as a carrier for systemic release of the active drug from the macromolecule while it circulates in the bloodstream and (2) as a carrier for on-site delivery which results from the release of the drug in the tumor as a consequence of accumulation of the copolymer in the tumor.