Insulin-like growth factor-I is a small growth factor known to signal in a variety of mammalian cells through the IGF-I cell surface receptor (IGF-IR). A unique feature of the IGF-I system is the regulation of this binding by soluble IGF binding proteins. Recent studies from our laboratory show that there is a pH dependence in the association of IGF-I with the cell surface in the presence of IGFBP-3 which suggested increased association of IGF-I with IGFBP-3 at low pH. We studied cell free interaction of IGF-I and IGFBP-3 as a function of pH using surface plasmon resonance (SPR) in order to understand the mechanism that causes the increased association. In our studies three different SPR instruments with different surfaces for immobilization of one of the binding partners were used: a Leica Bio-SPR 9000 with a low molecular weight carboxymethylated dextran (CMD) surface, a BIAcore 2000 with a high molecular weight CMD surface and a Leica SPR 2001 Alpha with a planar mixed self-assembled monolayer (mSAM) surface. Since the experimental system we used was transport sensitive, only the mSAM surface, under optimized conditions, produced results that fit to a single site model. Results suggest that use of CMD layers for immobilization of one partner of a high-affinity binding complex can result in transport limited binding for which simple analysis is inappropriate. Future studies are planned to expand the work with the mSAM surface to elucidate whether a significant difference between the binding parameters as a function of pH exists.