The concentration of Immunoglobulin G (IgG) in developing oocytes from normal chickens was measured by ELISA. The concentration was similar in oocytes ranging in size from 6 mm to 38 mm, which is in the last stage of development before ovulation. The concentration of immunospecific IgG was similar in corresponding serum and yolk samples in chickens immunised with human IgG, and a significant linear correlation between response of a specific antibody in the serum and corresponding egg yolk was found. The serum antibody level had to reach a certain level before specific antibody appeared in the egg yolk, and the egg yolk to a certain extent reflected the serum concentration of IgG over 6 to 7 days, because this is the time it takes for the developing oocyte to grow from 6 to 35 mm in diameter.