In order to establish a highly metastatic variant of a mouse colon carcinoma cell line (CT26), BALB/c mice were first subcutaneously injected with CT26 cells. Several weeks later, metastatic tumors in lungs were resected, mechanically dispersed into a single cell suspension and cultured in vitro until cells reached confluency. These tumor cells were then subcutaneously injected into new mice. After repeating this procedure five times, a highly lung metastatic cell line, denoted as LM17, has been established. The LM17 cells grow in vitro with or without serum, whereas parental CT26 cells require serum for their growth. The LM17 cells adhere to type I collagen or fibronectin stronger than CT26 cells do. The LM17 cells invade through Matrigel-coated basement membrane in greater number than CT26 cells. By gelatin zymography, LM17 cells showed higher proteinase activity than CT26. Furthermore, subcutaneous injection of irradiated LM17 cells infected with adenovirus harboring mouse GM-CSF gene prevents the growth and lung metastasis of pre-existing subcutaneous tumor. The injection of irradiated GM-CSF-producing LM17 cells after the surgical removal of pre-existing tumor also protected the occurrence of lung metastasis. These results suggest that this highly metastatic LM17 cell line could be useful for analysis of the lung metastatic mechanism and as the mouse GM-CSF gene therapy model.