Summary: An immunoglobulin was isolated from rabbit colostrum which differed from γG-immunoglobulin in its β-electrophoretic mobility, content of unique antigenic sites, higher sedimentation coefficient (10.8S) and higher carbohydrate content (3.2% hexose and 3.2% hexosamine). The colostral immunoglobulin appeared to be homologous with γA-immunoglobulins of other species. A yield of 3 to 4.5 mg of γA-immunoglobulin was obtained per milliliter of colostrum. The rabbit γA-immunoglobulin was used to prepare a specific anti-α chain reagent in the goat. The serum concentration of γA-immunoglobulin was found to be 55 to 217 µg/ml for various rabbits by quantitative precipitation with anti-α chain. The Ab4 allotypic sites controlled by the b locus were present on γA-immunoglobulin but allotypic sites controlled by the a locus (Aa1 and Aa2) could not be identified on this protein.