A mutant Sendai virus (SevMVC), which grows much better than its progenitor virus (SeVM) in cell culture, but, in strong contrast to SeVM, is totally avirulent for mice, has been described. SeVMVC contains two amino acid substitutions relative to SeVM, namely, F170S in the C protein and E2050A in the L protein. We have examined which substitutions were responsible for the above phenotypes by exchanging the C gene of our reference strain Z with those of SeVH (another reference strain), SeVM, and SeVMVC, in turn. We have found that the F170S mutation in the CMVC protein is responsible both for enhanced replication in cell culture and for avirulence in mice. Avirulence appeared to be due to restricted viral replication primarily after day 1, implicating some aspect of innate immunity in this process. The SeV C proteins thus appear to be required for multiple cycles of replication in mice.