We used a yeast two-hybrid system to identify binding partners for insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3). A partial complementary DNA encoding the carboxyl-terminal of fibronectin (FN), including the cell binding site, the heparin-binding domain, and the fibrin-binding domain, was identified in a screen of a human placental complementary DNA library. The interaction of IGFBP-3 with FN and the 40-kDa heparin-binding carboxyl-terminal fragment of FN was confirmed using Western ligand blotting. Both glycosylated and nonglycosylated IGFBP-3 bound to FN with a K(d) of approximately 0.3 nmol/L. IGF-I and IGFBP-1 had no effect on IGFBP-3 binding to FN. Competitive inhibition of IGFBP-3 binding to FN was observed in the presence of IGFBP-5 and heparin. The binding affinity of the immobilized IGFBP-3/FN complex for [(125)I]IGF-I (K(d) = 0.8 nmol/L) was similar to that of IGFBP-3 alone. The presence of IGF-I/IGFBP-3/FN ternary complexes in human plasma was demonstrated by coimmunoprecipitation of IGFBP-3 and [(125)I]IGF-I with anti-FN monoclonal antibody. These data indicate that FN may have a role in the transportation of IGFBP-3 and IGF-I in the circulation and the sequestration of these proteins in tissues.