An intermediate category is typified by the association between SV40 virus and nonpermissive mouse cells or between Polyomavirus and hamster kidney cells (BHK 21 ) (Dulbecco, 1968) in which the viral presence is signaled not only by altered cellular properties but also by the manufacture of virally specified antigens and the possibility of rescuing complete extracellular virus by fusion with permissive cells (Koprowski et al., 1967) .