With the aid of pure antibodies to polysaccharide of streptococcus, of group A, there was revealed by the method of indirect immunofluorescence with the use of pure antibodies to rabbit immunoglobulins (labeled with fluoresceine isothiocianate) localization of the cross-reacting antigen common for streptococcus and tissues of man and animals in the cells of surface epithelium of the ectodermal origin (the skin, salivary ducts, oral mucosa, esophagus, inferior portion of the rectum, uterine cervix, and the elements of the thymus epithelium). Detection of the cross-reacting antigen was of interest in connection with the study of the mechanisms of development of chronic pathological processes in man associated with streptococcus infection and investigation of the structure, function and pathology of the thymus.