The preliminary reports of others that a protein, designated IgA, distinct from other rat immunoglobulins is present in colostrum and serum have been confirmed. IgGa, IgGb, γ1 and trace amounts of IgM can be recognized in colostrum, but IgA is distinguishable in terms of unique antigenic characteristics and a concentration in colostrum relative-to-serum which vastly exceeds that of the other recognized rat immunoglobulins. Although it was not possible to demonstrate specific anti-hapten antibody within this class by the technique of radioimmunoelectrophoresis, the interaction of this protein with antiserum to rat Fab piece in agar double diffusion supports its designation as an immunoglobulin. For operational purposes the rat protein previously designated as IgA is now termed γ1, although it is not functionally homologous with the immunoglobulins so designated in other species.