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Cis-diamminedichloro platinum (II) (cis-DDP) and cis-diamminediaquo platinum (II) nitrate (cis-aq) were demonstrated to form complexes with dextran (dex) substituted with carboxymethyl (CM) groups at an average substitution ratio of 1 mole CM per 2 mole glucose units of dextran. The complexes were formed by reacting each of the two platinum (II) derivatives with carboxymethyl-dextran (CM-dex) at room temperature (RT) or at 37 degrees C in an aqueous solution. The complexing rate depended on temperature, ratio of platinum (II) compounds to CM-dex in the reaction mixture, and time of reaction. Experiments were performed with two CM-dex preparations, derived from dex T-10 (Mr-10,000) and from dex T-40 (Mr-40,000). Soluble cis-DDP and cis-aq complexes formed with CM-dex T-10 and CM-dex T-40 could carry up to 15 mole or 60 mole of the platinum (II) compounds per 1 mole CM-dex, respectively but higher complexing ratios resulted in complex precipitation. Reactivity of cis-aq with CM-dex was higher than that of cis-DDP. NaCl interfered with complex formation, but did not cause dissociation of already formed complexes. The binding of cis-DDP and cis-aq to CM-dex is, however, reversible since the drugs could be exchanged by other acceptors of higher affinity to platinum (II) such as O-phyenlenediamine, or DNA.