1. Sera from different embryonic stages of development and adult chickens were examined for embryonic-adult differences. 2. An antisera prepared in rabbits to 14 day embryonic chick sera revealed an antigen present in the early stages of embryonic development. The antigen reached a peak on day 14 and was not detectable 5 days post hatching. 3. Gel filtration studies in conjunction with immunodiffusion studies revealed the serum antigen to be in the 160,000 mol wt range. 4. Immunoelectrophoretic analyses of embryonic and adult sera revealed the sera components to increase in quantity and in numbers with development. The serum immunoelectrophoretic profile of newly hatch chick serum closely resembled sera from adult chickens. 5. Polyacrylamide gel electrophoretic comparisons of the sera of the developing chicks and the adult chickens revealed proteins specific to certain developmental stages as well as proteins specific to the adult serum. Qualitative and quantitative serum protein differences were found.