Both the amino-terminal and carboxyl-terminal domains of IGF binding protein (IGFBP)-3 are believed to contribute to high-affinity IGF binding. To investigate cooperativity in IGF binding by these domains, we expressed IGFBP-3 fragments 1- 88 (NBP-3) and 185-264 (CBP-3) as FLAG and hexahistidine- tagged fusion proteins, respectively. IGF-I and IGF-II bound to NBP-3 poorly and to CBP-3 with moderate affinities, approximately 1 liter/nmol. Coincubating the fragments in equimolar concentrations caused a significant cooperative increase in IGF binding, demonstrated by immunoprecipi- tation with IGFBP-3, FLAG, or hexahistidine antibodies. Equimolar NBP-3 CBP-3 bound IGF-II with an affinity (12.2 liter/nmol) only 4-fold lower than that of the IGFBP-3-IGF-II complex and IGF-I with an affinity (3.2 liter/nmol) 13-fold lower than IGFBP-3-IGF-I. Heterotrimeric complexes of NBP-3, CBP-3, and IGF, also demonstrated by affinity labeling, bound acid- labile subunit poorly. Coprecipitation assays with iodinated NBP-3 or CBP-3 indicated that the fragments cannot interact unless IGF is also present. Complexing with NBP-3 CBP-3 inhibited IGF stimulation of type 1 IGF receptor activity and IGF-II binding to the type II receptor. This study demonstrates that isolated amino-terminal and carboxyl-terminal domains of IGFBP-3 cooperate in the presence of IGFs to form high-affinity complexes that retain the ability to block IGF activity. (Endo- crinology 144: 2797-2806, 2003)