Interferon induction and the course of infection with paramyxovirus (Newcastle disease virus) and alphaviruses (Venezuelan equine encephalomyelitis, Sindbis, Semliki forest viruses) in human tonsillar cell cultures were studied in comparison with continuous Burkitt lymphoma, Raji, and Nawalva cells. The virus inducers stimulated production of dozens and hundreds IU50/ml of interferon in tonsillar cell cultures. No virus replication could be detected either in intact or phytohemagglutinin-stimulated tonsillar cells. Phytohemagglutinin treatment may lead to blocking of interferon production induced by Newcastle disease virus.