Trypsin-digested homogenates of callus of man, whole skin of guinea pig and rabbit were extracted with phenol water and the water phase (TPW water phase) preparations compared in serological tests. The TPW water phase from all three species sensitized erythrocytes for agglutination by normal human and animal sera irrespective of the species origin of the sensitizing antigen. Sera from individuals of the same species exhibited autospecificity in haemagglutination tests. Results of tests for inhibition of haemagglutination and of absorption experiments using pooled human serum indicated that the sensitizing antigens are species-specific. In addition to antibodies directed against these antigens, human serum probably also contain cross-reacting antibodies, indicating common stratum corneum antigens.