It was demonstrated that the development of experimental avitaminosis A in chicks led to secondary zinc deficiency. The balance of Zn in the chick became negative, while the Zn content of various tissues decreased. Thus in vitamin-A-deficient chicks the serum Zn content was 1258 (SD 26.3) micrograms/l which was considerably lower than 1652 (SD 97.8) micrograms/l in controls. Zn absorption was considerably reduced throughout the entire small intestine of vitamin-A-deficient chicks and most markedly in the ileal region. Within 72 h after retinyl acetate administration Zn absorption was fully restored in this region of the intestine. The 65Zn-binding capacity of soluble proteins, present in the supernatant fraction of ileal-mucosa homogenates of vitamin-A-deficient chicks, was found to increase 2.6 times by 72 h after the administration of a single dose of retinyl acetate. A vitamin-A-dependent Zn-binding protein (ZnBP), absent in vitamin-A-deficient chicks, was isolated from the ileal mucosa after their repletion with vitamin A. Competitive-binding studies (calcium, cadmium, copper) showed the protein to be highly specific for Zn ions. The molecular weight of ZnBP was 83 kDa. The association constant of the protein-Zn complex was 0.8 X 10(6)/mol. The protein was acidic with approximately 20% of its amino acid residues belonging to dicarboxylic acids. ZnBP was found to be a glycoprotein, and it contained hexose as a carbohydrate component. It is suggested that ZnBP is involved in the binding of Zn in the ileal mucosa of chicks.