Peritoneal macrophages were used to analyze host genetic control of MHV-4 infection in the 14 recombinant-inbred strains between susceptible SWR/J and resistant SJL/J progenitor strains. 4 strains were resistant to MHV-4 infection, while the remaining 10 strains were productively infected. Based upon the strain distribution pattern of susceptibility the gene locus for MHV-4 susceptibility is linked to the Svp-2 locus on the proximal end of mouse chromosome 7. This strain distribution pattern of susceptibility, and hence linkage, was determined to be identical for another MHV strain, MHV-A59. We have suggested that this locus-controlling susceptibility to MHV-4 and MHV-A59 be designated Hv-2.