Rat dopamine-producing nerve cells (1RB3AN27) and rat parotid acinar cells (2RSG) were immortalized by insertion of simian virus 40 (SV40) large T-antigen gene (LTa). Both of these cells divided and produced nuclear LTa in vitro. In order to assess the relationship between cell proliferation and expression of LTa in vivo, immortalized dopamine-producing nerve cells and parotid cells were grafted into the striatum and parotid gland of adult Sprague-Dawley rats, respectively. Grafted cells exhibited nuclear LTa at 1 day but not at 7 and 30 days after transplantation. At 30 days after transplantation, no tumor was found, and there was no evidence of cell division as determined by H and E staining. When the striatal areas containing the grafts were cultured, these cells did not express LTa at 4 days after plating; however, after 3 weeks, when most host cells were eliminated, the cultured grafted cells expressed LTa. After 3 months of culturing, only cells exhibiting LTa were present. These cells had the same morphology and divided with the same doubling time as 1RB3AN27 cells before grafting. Results suggest the presence of a LTa-inhibiting factor in vivo, and support the hypothesis that the expression of LTa is directly linked with proliferation of immortalized cells.