Insulin-like growth factor binding proteins (IGFBPs) are found both associated with cells and in extracellular fluids. Cell-associated IGFBPs increase [ 125 I]-IGF binding to cell monolayers, whereas extracellular (soluble, released) IGFBPs decrease binding. In the current study, we show that either IGFBP-3 or IGFBP-5 are the major forms of IGFBP released from monolayers of human GM10 fibroblasts, T98G glioblastoma cells and forskolin-treated bovine MDBK cells. IGFBPs represent the most abundant [ 125 I]-IGF-I binding site on GM10 and T98G cell monolayers, but 4-17% of the total cell-associated IGFBPs are released from the cell monolayer at 8ยฐC during their quantification. Most of the IGFBPs (. 70%) are released from MDBK cells. Quantitative estimates of [ 125 I]-IGF binding to the cell monolayers are altered because of the ability of the released IGFBPs to reduce the amount of radiolabeled ligand that is available to bind to the cell surface. Lanthanum (La 31 ) depresses IGFBP release from all three cell types (. 80% for GM10 and T98G cells and . 65% for MDBK cells). The effect was cation specific, noted with La 31 or Zn 21 but not with either Mn 21 , Sr 21 or Se 31 . The effect was also IGFBP specific; La 31 markedly depressed the release of IGFBP-3 and IGFBP-5, but had less of an effect on IGFBP-2 and IGFBP-4. Concomitant with a decrease in IGFBP-3 and IGFBP-5 release, La 31 caused an increase in [ 125 I]-IGF-I binding to cell-associated IGFBPs and type I IGF receptors. The released soluble IGFBPs have a three-to 20-fold greater affinity (K a ) for [ 125 I]-IGF-I compared to cell-associated IGFBPs. La 31 did not alter the affinity constants of cell-associated IGFBPs. In summary, we have identified a means to prevent loss of IGFBPs from cell monolayers during binding assays. This procedure will be useful in accurately quantifying the levels of IGFBPs on cell monolayers and in determining the role of cell-associated IGFBPs in controlling IGF activity. Retention of cell-associated low affinity IGFBPs may be important in controlling the size of the pericellular IGF pool and in regulating IGF-I access to the type I IGF receptor.