Experiments were undertaken to examine the effect of degradable starch microspheres (DSM) on the local distribution of 5-fluorouracil (5-FU) given by mesenteric arterial injection. Tritiated 5-FU (200 microCi) was injected into the inferior mesenteric artery of 8 perfused specimens of human large bowel containing adenocarcinoma and the concentration of 5-FU in local tissues and venous effluent was measured by scintillation counting. Metabolites of 5-FU were detected by radiochromatography. DSM (6 X 10(7)/mean diameter 40 micron) were injected in suspension with 5-FU (n = 7) and separately: before (n = 7) and after 5-FU (n = 7). Specimens given DSM retained 20% more cytotoxic than specimens given 5-FU alone (p less than 0.05). The concentration of 5-FU in tissues next to the primary tumour was increased when DSM were given in suspension with 5-FU, but higher concentrations were obtained when DSM were given last (p less than 0.05). Low concentrations were produced when DSM were given first. Concentrations in tumour tissue were not significantly increased by DSM.