Thy1 and CD44 are used as markers of hepatic stem and progenitor cells, respectively. We previously reported that transplanted Thy1+ and CD44+ cells isolated from galactosamine (GalN)‐treated rat livers could grow to form hepatic foci in rat livers treated with retrorsine following 2/3 partial hepatectomy (RET/PH). Although the number and the size of foci derived from Thy1+ cells were much smaller than those of foci from CD44+ cells and mature hepatocytes (MHs) at 14 days after transplantation, the livers with Thy1+ cells were bigger than those with CD44+ cells and MHs. In the present study we examined the reason why Thy1+ cell‐transplanted livers became large. Livers were immunohistochemically examined. Coculture of the cells isolated from a GalN‐treated liver and small hepatocytes (SHs) from a healthy liver was performed and the growth of SH colonies was measured. The number and size of small hepatocyte‐like progenitor cells, which are known as hepatic progenitors, were larger in the livers with Thy1+ cells than in the livers with CD44+ cells and MHs. The growth of SHs was enhanced when the cells were indirectly cocultured with Thy1+ cells. These results suggest that Thy1+ cells secrete substances that have the potential to activate the growth of progenitor cells in the recipient livers. This research was supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan.