Acute and convalescent sera from 77 patients with serologically confirmed influenza, measles and adenovirus infections and from 36 healthy controls were tested for the level of antibodies to Epstein-Barr (EB) virus. In the three groups of patients significantly higher titers of antibodies to EB viral capsid antigen (VCA) were found as compared to the controls. In 19 patients twofold or higher rise in antibody titers between the first and second blood sample was demonstrated. It is suggested that in patients with influenza, measles or adenovirus infections, involvement of lymphocytes leads to reactivation of EB virus and antibody formation is stimulated.