The pharmacokinetics of intraperitoneal cisplatin (DDP) administered with simultaneous intravenous sodium thiosulfate (STS) were investigated by a bioassay system using the phytohemagglutinin (PHA) stimulation assay of human peripheral blood mononuclear cells (PBM). Active DDP in the plasma, assessed by the bioassay system, was almost completely inactivated, when the level of STS in the plasma was more than 500 times higher at molar STS/DDP ratios than that of DDP. However, the peak level of active DDP in the peritoneal cavity was not significantly decreased. Fourteen patients with intraperitoneal carcinoma were treated with intracavitary DDP chemotherapy in combination with STS in this setting. Of 8 patients with malignant ascites who were evaluable for response, 4 patients experienced complete disappearance of ascites and the remaining 4 patients responded with apparent decrease in the volume of their ascites. No serious drug-associated toxicity was encountered.