In the chicken, maternal IgG is deposited in the oocyte yolk and subsequently transported intact by the foetal chick yolk sac into the chick circulation. Such transport appears to require a specific membrane-localized IgG receptor. Using the assay of 125I-yolk IgG binding, we have investigated the existence and properties of such receptors in yolk sac tissue disks. We find that: (1) There are specific, 125I-IgG binding sites on yolk sac tissue; (2) specific yolk 125I-IgG binding has a distinct pH optimum at pH 6.0–6.3; (3) the apparent equilibrium dissociation constant of specific 125I-IgG binding is 2–3 X 10(−6) M; and (4) heterologous proteins compete less efficently than yolk IgG for 125I-IgG binding.

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