Livers, sera, and erythrocytes of MAXX, BN, and Wistar rats were examined for an antigen (CRA) that cross-reacts with enterobacterial common antigen (ECA). Extracts of these tissues were tested for their capacity to reduce anti-ECA titers. By this parameter, CRA was present in certain, although not all, extracts of rat tissue. None of the extracts modified red blood cells (RBC) for agglutination by anti-ECA antisera, nor did MAXX liver extracts engender anti-ECA activity in rabbits. It was speculated that a repressor is present in rat tissue that affects both antigenicity and immunogenicity of CRA. To test this concept, extracts of mixtures of enterobacterial suspensions and liver homogenates were prepared. ECA in the presence of tissue did not modify RBC for anti-ECA activity as did ECA alone, nor did enterobacteria-liver extracts significantly reduce anti-ECA titers. Administration of ethanol-soluble fractions of enterobacteria-liver mixtures to rabbits did not elicit an anti-ECA antibody response as did ECA alone. The finding of CRA in certain rat tissues, sera, and erythrocytes may account, in part, for rats being refractory to immunization with ECA. It is proposed that repressors may mask or abrogate the expression of CRA in animal tissue.