Studies of the resistance patterns to infection with a murine cytomegalovirus in inbred strains of mice revealed the existence of resistant and susceptible strains. Resistance was found to be associated with possession of the H-2k allele at the major histocompatibility locus of the mouse. The F1 hybrid between a resistant strain (C3H/HeJ) and a susceptible strain (BALB/c) was found to have a resistance intermediate between that of both parents, indicating that the gene(s) controlling resistance is partly dominant. Susceptible BALB/c mice could be made resistant to lethal infection by pre-treatment with thioglycollate broth but not by pre-treatment with endotoxin or BCG. Resistant C3H/HeJ mice could be made susceptible to lethal infection by pre-treatment with cyclophosphamide.