Abstract The blood serum proteins of the two principal Antarctic penguins, the Adelie penguin (Pygoscelis adeliae) and the Emperor penguin (Aptenodytes forsteri), have been studied. Comparative studies were made with the domestic chicken (Gallus gallus), man, and another species of penguin, the Humboldt penguin (Spheniscus humboldti). Studies included starch-gel electrophoresis, disc electrophoresis, immunoeleetrophoresis, immunodiffusion, autoradiography, separations by gel filtration, and separations by ion-exchange chromatography. The serum proteins of Adelie and Emperor penguins separated on Sephadex G-200 into three main fractions very similarly to chicken serum proteins. The sera of both penguins, however, contained more multiple molecular forms of the serum transferrin than did chicken or human sera. The transferrins of all three penguins also had more alkaline mobilities on electrophoresis than either the chicken or human serum transferrins. The serum transferrin and the ovotransferrin of the Adelie penguin were found to be nearly identical electrophoretically. The high sialic acid content previously reported for Adelie penguin egg white was not reflected in comparatively high values in penguin blood serum. The blood sera of the three penguin species showed a strong immunological cross-reactivity by immunoelectrophoresis on rabbit antiserum to Adelie sera. Strong cross-reactions were found between Adelie penguin blood serum and Adelie egg-white proteins, particularly between the ovotransferrin and the serum transferrin.