An insulin radioreceptor assay (INS-RRA) and an insulin-like activity radioreceptor assay (ILAs-RRA) have been utilized to partially purify and characterize a protein from human amniotic fluid with ILAs-RRA reactivity. An acid-ethanol soluble protein with an apparent molecular weight of 34,500 daltons by calibrated Sephadex chromatography and an isoelectric point (pI) of 4.7 accounts for all of the ILA'S-RRA reactivity present in human amniotic fluid. Since this protein has been found to be a binding protein for ILAs, but not insulin, it has been termed amniotic fluid binding protein or AFBP. AFBP is reactive in a non-parallel manner in the ILAs-RRA and totally inactive in the INS-RRA. The activity of AFBP in the ILAs-RRA is thus to the competition of AFBP with the placental membrane receptor for the [125I]ILAs tracer employed in the ILA'S-RRA. AFBP inhibits the activity of employed insulin, in the INS-RRA, presumably by binding ILAs, while being inactive itself. In two biological assays studied to date, the rat epididymal fat pad assay and the rabbit chondrocyte sulphation assay, AFBP also inhibits the activity of added ILAs. These observations raise the possibility that binding protein(s) for insulin-like peptides may function as inhibitors of their bioactivity in different physiologic and pathologic states. The relation of AFBP to binding protein(s) in human plasma remains to be clarified.