The relative importance of the humoral immune response to various antigenic sites on the glycoproteins C and B (gC, gB) of herpes simplex virus (HSV) was evaluated in BALB/c and DBA/2 mice passively immunized with monoclonal antibodies (MoAbs) and then challenged with lethal dose of infectious virus. Eight MoAbs to three topographically distinct antigenic sites on gC and eight MoAbs to two distinct antigenic sites on gB were selected. The results indicated that any antigenic site on gC and gB contains epitopes for the protective immunity. However, individual MoAbs to different epitopes of the same antigenic site (i.e. antigenic site III on gC, and antigenic site II on gB) varied extremely in their protective ability. The protection did not correlate with the virus neutralization in vitro whereas it correlated significantly with the immune cytolysis in the presence of complement. The information about protective epitopes is essential for understanding the immunology of HSV infection at molecular level and may have implications for the design of HSV vaccine.