Human adenocarcinoma cells injected into the peritoneal cavities of BALB/c nude mice (nu/nu) induced ascites carcinoma. The inoculant was obtained from subcutaneous tumors produced in nude mice by an injection of ascites cells from a patient with carcinomatous peritonitis caused by mucinous adenocarcinoma of the stomach. An ascitic fluid began to accumulate 45 days after inoculation and reached the maximum volume within 120 days. Dispersed stomach cancer cells in the ascites could be serially transplanted in nude mice in an ascites form. The morphology of these cells was similar to that of the original cells in the ascitic fluid of a patient with carcinomatous peritonitis.