Plasma levels and serum protein binding of cis-diamminedichloroplatinum(II) (cis-DDP) or cis-diamminediaquoplatinum(II) (cis-aq) complexed to carboxymethyl-dextran (CM-dex) with a molecular weight of 10,000 (T-10), 40,000 (T-40), and 250,000 (T-250) were investigated in BALB/c mice. Levels of active drug in the circulation after the i.v. or i.p. administration of the free or complexed drugs, as well as the loss of drug activity due to serum protein binding following incubation with mouse serum, were monitored by an antitumor in vitro assay using a drug-sensitive tumor cell line. Following i.v. injection of the complexes, active platinum(II) was maintained in the circulation at higher levels and for a longer period, whereas the free drug disappeared rapidly. The rate of disappearance of the complexed drug from the circulation was markedly influenced by the molecular size of the carrier CM-dex, since the retained amount of drug was considerably higher with the T-40 and T-250 complexes than with the T-10 complex. An i.p. injection resulted in a rapid and transient appearance of low levels of the free drugs in the blood, whereas in the case of the complexes, transport to the circulation was slower and their maintenance in the blood system was markedly higher. Serum protein binding was much slower with CM-dex-complexed drugs (regardless of the molecular size of the CM-dex carrier) than with the free drugs.