In previous studies it was observed that red blood cell membranes (RBC-M) induce in rats the formation of antibodies against RBC-M components extracted with detergents. The same reaction was observed by employing sera from rats treated with RBC-M extract prepared with a low ionic strength saline solution and containing especially spectrin. This and the lack of significant changes in the hematologic picture suggested that the antibodies were directed against an inner RBC-M component, namely spectrin. In the present research we tried to verify if the spectrin would be involved in the immune reaction. For this, gel filtered serum fraction containing IgG from rats treated with entire RBC membranes or with spectrin extract, in adjuvant, has been employed in immune reactions against RBC membrane components separated by means of SDS-PAGE. Immunoblotting and two-dimensional electroimmunodiffusion techniques have been used. With both the techniques the IgG fractions from sera of rats treated with RBC-M and of rats treated with RBC-M extract reacted with two bands of spectrin, especially with band 1. A very mild reaction appeared employing IgG of rats treated with only adjuvant. The results demonstrate the immunogenicity and antigenicity of homologous spectrin; they are supported by results of other Authors which demonstrated the presence of natural antibodies against spectrin. It is suggested that the precipitating antibodies represent an enhancement of a natural, normally operating immune reaction against inner RBC-M components. At present its rôle remains to be defined.