Cell suspension of a human breast cancer cell line (Hattori line) was injected intraperitoneally into an athymic nude mouse to produce ascites form breast cancer (peritoneal carcinomatosis). Subsequent serail transfers of cancer cells in ascites were also successful in mice. All male and female nude mice injected 1 X 10(7) tumor cells died of accumulation of ascites after a latency period averaging 4 weeks, with one exception which died of a wasting disease. Multiple lung metastases were observed in some mice. The tumor cells retained cytological characteristics of the original cell line, and histology of the infiltrating tumor in the peritoneum and omentum was that of poorly differentiated adenocarcinoma. Differentiation not only toward acinar or duct lining cells but also toward myoepithelial cells was observed by histochemistry, immunohistochemistry, and electron microscopy.