Abstract 1. 1. The egg-white proteins of the Antarctic Adelie penguin ( Pygoscelis adeliae) were studied by a variety of physical, chemical, biochemical and immunological methods. 2. 2. As compared to other avian whites, the penguin egg white was unusually high (4–5 per cent) in sialic acid and quite low in lysozyme (approximately 0.05 per cent). The sialic acid was mainly present in combination with constituents of isoelectric point between pH 3.5 and 6. 3. 3. The major constituents, ovalbumin, conalbumin and ovomucoid, had properties generally similar to those found in various other egg whites. The conalbumin existed in four to five multiple molecular forms as indicated by starch-gel electrophoretic analysis. The ovomucoid had dual inhibitory activity against both trypsin and chymotrypsin. 4. 4. On the basis of comparison of the properties of the egg-white proteins, the Adelie penguin is much more closely related to the lower water fowl than to the ratites.