Herpes simplex virus type 1 and type 2 antigens extracted from infected cells with use of nonionic detergent produce a number of distinct immunoprecipitates in crossed immunoelectrophoresis (CIE) with hyperimmune rabbit antibodies to herpes simplex virus. The analytic power of CIE combined with the use of single immunoprecipitates made it possible, for a number of individual antigens, to determine (1) the immunogenicity of the antigens in experimentally infected animals and naturally infected humans, (2) the distribution of type-common and type-specific antigenic determinants on the antigens, (3) the biochemical structures of the antigens, (4) the localization of the antigens on the infectious virus particle and in infected cells, and (5) the ability of the antigens to act as targets for neutralizing antibodies and for antibody-complement and antibody-dependent cellular cytotoxicity. Two herpes simplex virus type-common, membrane-bound glycoproteins, Ag-ll (gA + gB) and Ag-8 (gD), which are localized on the surface of the viral envelope and in the membranes of infected cells, are the principal immunogens in infected humans. The monospecific antibodies against these two antigens neutralize the virus and cause cytolysis of infected cells in the tests mentioned above. Antibodies to glycoproteins carrying herpes simplex virus type-specific antigenic determinants also are present in infected humans. These antigens can be utilized in herpes simplex virus type-specific human serology, and the corresponding mono specific antibodies can be used for the typing of herpes simplex virus isolates.