ALLOTYPIC specificity of rabbit γ-globulins differentiating individuals of the same species, was first described by Oudin1. This kind of specificity is defined by means of reactions between rabbit iso-precipitating sera and the serum of individuals of the same species. The iso-precipitating rabbit sera used for differentiating rabbit γ-globulins were obtained in two ways: (1) by injecting rabbits with immune complexes composed of heterologous antigen (human, equine, sheep and hen erythrocytes) and their corresponding rabbit immune antibodies; (2) by injecting rabbit with suspensions of rabbit globulins obtained by precipitation with aluminium potassium sulphate. In both cases it was necessary to take into account differences in γ-globulin characteristics between the rabbit donor of hetero-agglutinins or globulins and the rabbit donor of iso-precipitins.