A fast (7 S γ1) and slow (7 S γ2) 7 S γ-globulin system in the hamster has been described. Specific antisera to hamster 7 S γ1 and 7 S γ2 were prepared, and the antigenic relationship of the papain and pepsin digests of 7 S γ2 were compared to each other and to the 7 S γ1-globulin. Antigenic determinants common to the 7 S γ1- and 7 S γ2- globulins were located in the Fab and F(ab')2 piece of 7 S γ2, whereas antigenic determinants unique for the 7 S γ2 whereas antigenic determinants unique for the 7 S γ2 molecule were in the Fc piece of 7 S γ2. The molecular size of 7 S γ1-globulin was shown to be similar to that of the 7 S γ2-globulin (s020, w 7.2) and the antigen-binding capacity of the 7 S γ1-antibody was not detectably affected by 2-mercaptoethanol or heat treatment.