The contributions of the Fc fragment of virus-specific antibody in the resistance of mice to peripheral herpes simplex virus infection were investigated. Rabbit anti-herpes simplex virus-specific F(ab')2 fragments prepared by pepsin digestion of immune immunoglobulin G (IgG) were found to be inactive in complement-mediated cytolysis while retaining their capacity to neutralize virus infectivity in vitro. When F(ab')2 fragments were passively transferred either before or simultaneously with virus inoculation, they were as efficient as intact IgG was in protecting animals from virus challenge. However, if passive transfer was delayed until 8 h after herpes simplex virus infection, only IgG antibody was protective. The loss of protective activity could not be attributed to a rapid disappearance of F(ab')2 fragments, because comparable levels of F(ab')2 fragments and IgG antibody were maintained in the blood of recipients during the time that antibody mediated its protective effects. The inability of F(ab')2 subunits to activate complement was also not a factor, because complement-deficient A/J mice and complement-sufficient SJL/J mice recovered from herpes simplex virus infection after the passive transfer of IgG. We concluded that the Fc component of the antibody molecule is needed to resolve intracellular infection and that the mechanism by which antibody mediates recovery remains undefined but does not appear to involve virus neutralization or complement activation.