Polystyrene-based ion exchange resins were evaluated as a carrier system for the sustained delivery of cytotoxic agents. A batch ion exchange procedure was developed for loading of adriamycin (Adr) and novantrone (Nov) onto cation exchange resins and 5-fluorouracil (5-FU) and floxuridine (FUDR) onto anion exchange resins. Loading proceeded to near resin capacity in the case of Adr, 5-FU and FUDR. A closed circuit in vitro drug release system was developed to measure rates at which resins released entrapped drug and reached equilibrium drug concentrations. A large degree of variation in the equilibrium drug concentrations was observed between the different drug systems (6 × 10-7 M-1.7 × 10-3 M) but the rate at which equilibrium was attained was similar for all drugs. Adr resinate demonstrated equilibrium drug concentrations close to blood levels associated with conventional therapeutic doses. Thus Adr resinate is regarded as having potential for tumour-targeted drug delivery.