Three categories of cell lines are described which differ with respect to their permissiveness to mouse hepatitis virus (MHV), strain A59. Fully permissive L-2 cells gave rise to 100- to 1000-fold higher numbers of infectious centres than did semi-permissive LM, LM-K or C-1300 cells, whereas non-permissive Vero or C-6 cells were refractory to MHV infection. On an infected cell basis, semi-permissive cells (LM, LM-K or C-1300) were as efficient in replicating viral RNA, protein and progeny virions as fully permissive L-2 cells. This result suggested that LM, LM-K and C-1300 cells were deficient in their ability to permit full expression (as compared to L-2 cells) of an early event in MHV infection. Assays of radiolabelled MHV binding to cells of all three categories (L-2, LM, LM-K and C-6) and of infectious MHV binding to L-2 and LM-K cells showed no correlation between virion binding and degree of permissiveness to MHV infection. Internalization of MHV virions into L-2 and LM-K cells, as assayed by proteinase K-resistant infectious centres, showed that, in both cases, maximum virion uptake was complete by approximately 40rain post-inoculation. Direct assays of infectious virion uptake showed similar numbers of internalized viruses (only a