The proliferation of primary cultured rat hepatocytes was observed in serum-free modified Dulbecco's modified Eagle's medium supplemented with 10 mM nicotinamide and 10 ng/ml of epidermal growth factor. These proliferating cells were mainly mononucleate and formed small-cell colonies after Day 4. The small cells in focal colonies were surrounded by typical hepatocytes and were stained immunocytochemically with anti-rat albumin and anti-cytokeratin 8 antibodies. This suggests that the cells in the small-cell colonies were derived from hepatocytes. The frequency of appearance of small-cell colonies with age was examined by the use of primary cultured hepatocytes isolated from the livers of rats between the ages of 3 wk and 90 wk. In the cells from 4- to 5-wk-old rats, about 58 colonies per 1000 attached cells appeared 144 h after plating; the number of colonies rapidly decreased to about 25 in 6- to 8-wk-old rats. In adult rats, about 17 colonies were seen, and only about five colonies were observed in rats more than 80 wk old.