The biotin and biotin-binding protein contents of egg yolk, egg albumen, and hen plasma were determined on eight groups of four turkey hens each that had been fed diets ranging from less than 10 to 3,475 micrograms available biotin per kilogram. Biotin deposition in the yolk was strongly dependent upon available dietary biotin below 100 micrograms/kg. Between 100 and 1,000 micrograms/kg the amount of biotin deposited in the yolk increased slightly and was directly related to and limited by a biotin-binding protein that transferred biotin from the plasma to the yolk. Over the entire dietary range, biotin deposition in yolk was proportional to the total biotin concentration in the plasma. In contrast, biotin deposition in the albumen, which was proportional to dietary biotin, increased several-fold over a very narrow range of plasma biotin concentration (56 to 62 micrograms/L). When dietary available biotin exceeded 160 micrograms/kg, there was more biotin deposited in the albumen than in the yolk. Although the concentration of unbound biotin in plasma is low, it appears to be the component of plasma biotin that is rapidly scavenged by avidin in the oviduct. It seems likely that avidin-bound biotin is available to the turkey embryo.