We have studied the expression of insulin-like growth factor type II (IGF-II) and its autocrine role during the proliferation and differentiation of the CaCo-2 colon carcinoma cell line. IGF-II RNA levels were high in proliferating cells and decreased by more than 10-fold when cells ceased to proliferate and differentiated. Immunoreactive IGF-II protein was high in the conditioned media of proliferating cells and decreased 20-fold in the media of differentiated cells. Reduced IGF-II expression was associated with a decrease in IGF-I receptor number that was high in proliferating cells (approximately 80,000 binding sites/cell) and reduced by 4-fold in differentiated cells. Exogenously added IGF-II was able to stimulate proliferation of serum-deprived cells in a dose-dependent fashion. IGF-II acted through the IGF-I receptor, since both basal and IGF-II-stimulated cell proliferation was inhibited by the monoclonal antibody alpha-IR3, which blocks the binding sites of the IGF-I receptor. The inhibition of CaCo-2 basal cell growth by the alpha-IR3 antibody suggests that IGF-II may act as an autocrine growth factor for these cells.