Intraperitoneal injections of OK-432, originated from Group A streptococcus pyogens of human origin, were administered to 77 patients with ascites caused by cancer of the digestive tract. Complete disappearance of effusion was observed in 43 cases, its reduction in five out of 77. They received cytologic examination of ascites daily before and after OK-432 injection. One of these patients, a 77-year-old man with carcinomatous peritonitis due to gastric cancer, showed an interesting phenomenon after OK-432 injection. All adenocarcinoma cells in his ascites disappeared at least with 36 hours after OK-432 injection with increasing number of intraperitoneal neutrophils. In addition, neutrophils collected from his ascites or peripheral blood showed cytostatic effect on his ascites-derived tumor cells in vitro. His neutrophil-depleted intraperitoneal cells, however, had no significant effect on DNA synthesis of his tumor cells in vitro. OK-432 itself had no significant effect on DNA synthesis of his tumor cells in vitro. This report describes a patient in whom OK-432-induced neutrophils may play an important role in his tumor cell destruction in ascites.