IDF45 (inhibitory diffusible factor) a mouse insulin-like growth factor binding protein (mlGFBP-3) has been shown to 100 percent inhibit DNA synthesis stimulated by serum in chick embryo fibroblasts (CEF). Our previous results suggested that this large inhibition by IDF45 of serum stimulation was not just the result of its inhibitory activity toward IGF present in serum. The addition of Mn2+ (10(-3)M) in the incubation medium enables us to show the presence of numerous binding sites per cells (about 60,000) of mlGFBP-3. However the dissociation constant (10(-8)M) indicated that this mouse IGFBP-3 bound to the membrane with low affinity. These findings lend new support to the assumption of the bifunctional property of IGFBP-3, which would have an effect outside the cell (binding of IGF in the medium) and another effect within cells or on the surface.