Several distinct insulin-like growth factor binding proteins (IGFBPs) are present in tissues and fluids of the developing and adult eye. However, the mechanism(s) involved in the regulation of ocular IGFBP levels is unknown. We have now identified an endogenous factor in vitreous and aqueous humors that, when activated by sodium dodecyl sulfate (SDS), abolishes the capacity of specific low molecular weight IGFBPs (i.e. 24-30 kDa) to bind IGF as assessed by western ligand blotting. In contrast, IGF binding to the 46 and 32 kDa IGFBPs (IGFBP-3 and IGFBP-2 respectively) is not affected by the SDS-activated inhibitory factor (IF). Maximal activation of the IF occurs at an SDS concentration of approximately 0.015%. Incubations in the presence of the serine-proteinase inhibitor aprotinin result in marked inhibition of IF activity. Preliminary characterization by ultrafiltration suggests that the IF is large (< 100 kDa) and/or that it is present in a complex. The finding of a factor, most likely a serine proteinase, that specifically abolishes IGF binding to low molecular weight IGFBPs suggests a mechanism for regulating the levels of these IGFBPs and thus the functional activities of IGFs in ocular fluids under normal and/or pathological conditions.