Immunoglobulin G from human, horse, sheep, pig, cow, chicken, rabbit and dog blood serum was analyzed for its carbohydrate composition by gas chromatography, using alditol acetate derivatives of sugars released by acid hydrolysis. Fucose, mannose, galactose, glucosamine and sialic acid were present in IgG of all species studied. Most of the carbohydrate was found in the H chains. Only small fractional molar quantities of the carbohydrates were present in L chains of all species except chicken. Chicken IgG was also distinctive in that it contained 4.5% total carbohydrate as compared with only about 2% in the other species studied. The presence of less than one residue of fucose, galactose and sialic acid per H chain indicates heterogeneity of carbohydrate composition. © 1971.