Chicks depleted of hepatic vitamin A reserves were maintained on diets supplemented with either 2 ug retinol, 2 ug all-trans retinoic acid (RA), or 0.2 ug retinol/g of diet. Chicks were vaccinated with Newcastle disease virus (NDV) at 2 weeks of age. Serum samples were obtained weekly for determination of serum IgG, IgM, and IgA concentrations. At 7 weeks of age, RA-fed chicks were further divided into three treatment groups. One group remained on the RA control diet (2 ug/g of diet); the other two groups were fed either 0 or 0.2 ug RA/g of diet. Chicks fed 0.2 ug retinol/g of diet were changed to a vitamin A-free diet. Chicks fed the retinol control diet (2 ug retinol/g of diet) remained on this diet throughout the experiment. At 2, 4, or 6 days after final diet changes were made, chicks were revaccinated with Newcastle disease virus;At 7 weeks of age, serum IgG, IgM, and IgA concentrations were significantly greater in the retinol deficient and RA control chicks as compared with retinol control chicks. Eight days after the second NDV vaccination, serum IgG and IgM concentrations were significantly higher in chicks fed a retinol or RA deficient diet as compared with chicks fed either 2 ug of retinol or RA/g of diet, respectively. Serum IgA concentrations were greater in retinol deficient chicks after revaccination but lower or similar amounts of IgA were detected in the serum of RA deficient chicks as compared with controls. Despite an increase in serum immunoglobulin concentrations, serum antibody titer in response to the second NDV vaccination was significantly decreased in chicks fed a vitamin A deficient diet as compared with controls;Secretory IgA (SIgA) concentrations in bile, intestine, and trachea 8 days after revaccination were lower in vitamin A deficient chicks as compared with controls. In contrast to the low SIgA concentrations in tissues, significantly greater concentrations of IgM and IgG were found in intestine and trachea of vitamin A deficient chicks than in controls;Serum IgG, IgM, IgA, biliary IgA, and serum antibody responses were greater in chicks fed diets supplemented with 2 ug RA/g of diet than in chicks fed an equivalent amount of vitamin A in the form as retinol.