LMH chicken hepatoma cells show type 1 IGF receptors and a 28 kDa IGF-binding protein (IGFBP) on their membranes. They also secrete large amounts of the 28 kDa IGFBP. Following overnight incubation in serum-free medium, human IGF-I was markedly less effective than insulin in stimulating amino acid (AIB) uptake. Chicken and human IGF-I were equipotent, consistent with their equipotency in inhibiting [125I]IGF-I binding to wheat germ agglutinin-purified IGF receptors or membrane solubilized IGFBP. When cells were supplied with fresh medium, cell-associated IGFBP were unaffected, but the level of soluble IGFBP was largely reduced. This potentiated the effect of IGF-I on AIB uptake. The effect of chicken Long-[Arg3]-IGF-I, which exhibited low affinity for the IGFBP, was unchanged. In fresh or conditioned medium, this analog was more potent than IGF-I, suggesting that both soluble and membrane-bound 28 kDa IGFBP inhibited the effect of IGF-I.