In the chicken, maternal antibodies are transferred into the egg and subsequently transported into the developing embryo. IgG is the primary immunoglobulin isotype of the egg yolk, while IgM and IgA are mainly found in the albumen. However, considerable amounts of IgM and IgA of unknown origin are found one day prior to hatching in the yolk sac. These antibodies are not synthesized de novo by the embryo proper, thus pointing to a transfer from the albumen into the egg yolk during development. To further address this question, 125I labelled chicken IgA was injected into the albumen of freshly laid eggs. On day 21 of embryonic development, 125I-IgA was found in the yolk sac content. On average (n = 6) 36.2 +/- 7.2% of the injected radioactivity was recovered from this compartment and shown to be associated with IgA. Comparison of total IgA in the albumen of freshly laid eggs with the amounts of IgA in yolk sac content showed similar results with a 44% transfer rate. An increase of the IgA concentration in the yolk sac was first detectable between days 14 and 16 of embryonic development. These data clearly show that IgA is transferred from the albumen into the yolk sac, most likely by a transport across the yolk sac membrane.