The insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-II/MPR) has a specific binding site for IGF-II, a fetal mitogen. In rodents, IGF-II/MPR expression declines dramatically after birth. To see whether such developmental regulation occurs in humans, we studied the ontogeny of the soluble form of IGF-II/MPR in amniotic fluid (AF) and serum. Phosphomannan-affinity purified AF IGF-II/ MPR was a single band of approximately 220 kDa, like the band in serum, and it bound IGF-II with affinity identical to that of the membrane-associated form. By quantitative immunoblot, the soluble IGF-II/MPR in serum and AF was found to undergo developmental regulation that parallels that of the rodent, although it is much less pronounced quantitatively. The highest levels are seen in midgestation, decreasing at term in both serum and AF. In serum, they further decline to one-third of the preterm levels by adulthood. As part of characterizing AF IGF-II binding, we also show that the prominent high-molecular mass IGF-II-binding protein in preterm AF is GPC3, a protein of the glypican family, recently cloned because its mutations predispose to Wilms' tumor. For the first time, we show that IGF-II binding to this protein is saturable, and therefore specific. These findings should promote understanding of the role of IGF-II and its binding proteins in human development.