The humoral and cellular immune responses of rabbits and guinea pigs to the envelope-associated antigen of herpes simplex virus type I were studied. Neutralizing antibody (at high titer) and lymphocytes reactive to herpes simplex virus were detected in both guinea pigs and rabbits after immunization with the antigen. In a standard assay of cellular immunity to herpes simplex virus, the antigen stimulated blast transformation of herpes simplex virus-reactive splenic lymphocytes in vitro. Furthermore, immunization of rabbits with the envelope-associated antigen protected the animals from a lethal dose of live herpes simplex virus. Thus an antigen of herpes simplex virus can be prepared which contains neither infectious nor noninfectious viral particles and which stimulates immunity to the virus in laboratory animals.