We have analyzed, in parental and c-jun-transfected rat liver epithelial (REL) cells, the capacity of the insulin-like growth factors (IGF1 and 2) to stimulate growth under anchorage dependent and independent conditions and the role of the c-jun protein in the response to these factors. Although the increase of the proliferation of cells in monolayer was similar for the two lines, only the c-jun-transfected cells formed colonies in soft agar after IGFs exposure. We showed that IGF1 and 2 did not modify the expression of c-fos or c-jun proteins during short-time exposures but both factors enhanced c-jun phosphorylation which suggests that this phenomenon could be involved in IGFs-regulated gene expression. Moreover, we showed that both the overexpression and the increase of phosphorylation status of c-jun protein were necessary to transform REL cells.