Twelve patients with malignant ascites caused by gastric cancer were treated with intraperitoneal injections of a streptococcal preparation, OK-432. All had resolution of the ascites after OK-432 treatment. Neutrophils, macrophages, and lymphocytes increased in number in ascitic fluid samples. Some of the OK-432-induced inflammatory cells were attached to tumor cells. The absolute number of tumor cells decreased as the number of infiltrating inflammatory cells increased. Infiltrating lymphocytes were mainly E rosette-forming cells. Infiltrating macrophages were in an activated state. The infiltrating neutrophils, lymphocytes, and macrophages could inhibit DNA synthesis of the patient's own tumor cells in the ascitic fluid after OK-432 injection, but not before the injection. These results indicate that OK-432-induced neutrophils, lymphocytes, probably T cells, and activated macrophages may play an important role in tumor cell destruction in ascites. Moreover, as the number of tumor cells decreased, the ascitic fluid protein levels decreased. Decrease of the ascitic fluid protein level may suppress further accumulation of ascitic fluid, and the low protein level in ascitic fluid is likely to facilitate the reabsorption of the fluid into the bloodstream.