A new dosage form of cisplatinum (CDDP), lactic acid oligomer microspheres incorporating cisplatinum (CDDP-ms), is designed to slowly release 70% of contained CDDP. CDDP-ms's acute toxicity is as low as 57% of the toxicity of CDDP aqueous solution, and its therapeutic efficacy is statistically significantly strong as compared with that of CDDP aqueous solution, when examined with experimental peritoneal carcinomatosis induced by mouse M5076 ovarian sarcoma. Clinical trials were carried out in 10 patients with malignant ascites (gastric cancer 6, pseudomyxoma peritonei 2, colon cancer 1, pancreas cancer 1) and in one patient with pleural effusion (lung cancer). CDDP-ms at 100 mg/person in terms of CDDP was injected at bolus into the affected cavity. In the 10 patients with ascites, 7 responded completely, two partially and one did not respond. The patient with pleural effusion responded partially. The response rate was 91%. Five of the 11 patients complained of temporary nausea or vomiting. In 5 patients fever higher than 38 degrees C was seen. No other side effect such as kidney, nor liver-damage or blood cell count abnormality was noted.