The possibility that viruses are involved in the pathogenesis of aphthous lesions was investigated. Immunofluorescent-antibody tests showed that antigens of adenovirus and herpesvirus are present in cells from aphthous lesions and lymphocytes from persons with aphthae more frequently than in cells from normal persons. Antigens of adenovirus type 1 and herpesvirus caused blastogenic transformation in a significant proportion of lymphocytes from aphthous patients. The findings suggest that virus-carrier lymphocytes may be an important factor in the development of certain viral diseases.