The effect of adenovirus infection on human peripheral lymphocytes has been studied in vitro. Virus antigens were found to multiply in the cell cultures but no infective virus was formed. On infecting the lymphocytes after stimulation with phytohaemagglutinin, infective virus was formed though in low titre. Adenovirus infection decreased the E-rosette forming activity and the response to phytohaemagglutinin of the lymphocytes. The immunomodulating drug levamisole failed to influence significantly the effect of the virus. Adenovirus thus causes several changes in the characteristics of peripheral lymphocytes which may lead to a disturbance of the immune function.