An ascitic subpopulation (Anip) was isolated from human lung adenocarcinoma cell line AGZY-83-a through serial intraperitoneal passages in nude mice. Anip manifested a much higher metastatic potential than the parent line. Spontaneous lung metastases occurred in 95% of the mice xenografted intraperitoneally, and the diffuse pattern of the metastases in the lung was very impressive. Anip also differed more or less from the parent line in morphology, growth, rate, electrophoretic mobility, platelet aggregating activity and noticeably in the chromosome histogram. The present model may be used in the study of tumor invasion and metastasis, as large amounts of well dispersed metastatic human carcinoma cells could be reproduced by ascites and much information about the biology of invasion and metastasis of human tumor cells in nude mice could be obtained.