The subcutaneous growth potential of a Friend virus-induced murine leukemia cell line (FBL-3) passaged in the ascites form was found to change, depending on the interval of time spent in the peritoneal cavity. Injection sc of the original stock of FBL-3 (growth continuously in vitro) into C57BL/10 mice produced transient tumors uniformly rejected by days 20-40. The tumor cells passaged in the ascites form for 7 days behaved like the in vitro-grown cells, whereas ascites cells harvested at 14 days produced progressive lethal subcutaneous tumors in 50-70% of the recipients. The change in subcutaneous growth was reversible simply by alteration of the ascites passage schedule. Day 7 ascites cells (regressors) were converted to progressors by ip passage for 14 days, and day 14 ascites cells (progressors) were converted to regressors by ip passage for 7 days. The difference in growth potential between day 7 and day 14 ascites cells was not due to effects of nonneoplastic host cells accompanying the tumor cells in the ascites population, because neither dilution of nonneoplastic cells by in vitro culture nor selective killing of H-2a/b host cells by anti-H-2 serum and complement altered the subcutaneous growth behavior or either day 7 or day 14 ascites cells. These results indicated that the change in the growth potential of FBL-3 occurred at the level of the tumor cells. However, no quantitative differences were observed in the expression of serologically detectable tumor-associated antigens by these two populations. Possible mechanisms for this change in transplantability were considered.