A quantitative method for immuno-electron microscopy (IEM) of herpes simplex virus (HSV) is described which is based on comparative particle counts of residual, free virions remaining in suspension after the incubation of HSV virus suspensions with antiserum. This method has been used for the characterization of those antigens of the viral nucleocapsid and envelope which reacted with selected rabbit hyperimmune and human convalescent antiserum. The presence of viral-specific antigens in the nucleocapsid and the envelope was demonstrated by agglutination reactions with the rabbit anti-viral sera used; host antigens could not be detected. One antigen of the viral envelope reacted with rabbit hyperimmune antisera, but not with human convalescent serum; the possible significance of this antigen is discussed.