Survival of the chick depends upon the transfer of maternal IgG from the egg yolk into the fetal circulation. Radiolabeled chicken IgG binds specifically to both fixed and unfixed yolk sac tissue from day 7 or 8 of embryogenesis through at least day 18. This binding is saturable, Fc-specific, pH-dependent, and reversible. Chicken Fc fragments and some heterologous IgGs compete with IgG binding, whereas bovine serum albumin, phosvitin, conalbumin, chicken IgM, and chicken Fab fragments do not. Eight-day embryos appear to have only a low affinity receptor, KD = 3.4 x 10(-7) M, but 18-day embryos have an additional high affinity receptor, KD = 3.0 x 10(-8) M. The membrane density of the low affinity receptor remains essentially constant throughout development.