The role of interferon alpha/beta (IFN) induction by the immunomodulators pICLC and CL246,738 was investigated in CD-1 mice infected with murine cytomegalovirus (MCMV) or herpes simplex virus type 2 (HSV-2). Mice were treated with either normal sheep serum or anti-alpha/beta IFN antiserum, inoculated with the immunomodulators, and infected with virus. Because anti-IFN treatment also decreased natural resistance to HSV-2 and MCMV, two viral challenge doses were used to ensure that the mice with control serum or anti-IFN antiserum received biologically equivalent infections. Antiviral protection of pICLC and CL246,738 against HSV-2 infection was completely abrogated by treatment with anti-alpha/beta interferon antiserum. Mice treated with pICLC also lost antiviral protection against MCMV when interferon alpha/beta was depleted. These results indicate that induction of interferon alpha/beta appears to be a major mechanism for both natural resistance and pICLC-induced antiviral protection against MCMV and HSV-2 herpesvirus infections.