Rabbit antisera were prepared against the purified glucoamylases I and II ofAspergillus niger. Relationships between the two enzyme forms were investigated by using the antisera in immunodiffusion and immunoinhibition experiments. Both the forms of glucoamylase gave a single continuous precipitin band demonstrating very close structural resemblance. They gave almost identical immunoprecipitation patterns and had the same equivalence points indicating that the two forms ofA. niger gluoamylases were immunologically identical. The enzyme treated with periodate was immunologically identical with the controls and had slightly less enzyme activity but showed greatly reduced stability on storage at 4‡ C.