Attempts at diagnosis of Herpesvirus hominis encephalitis by isolation of virus and a histologic search for type A intranuclear inclusion bodies from biopsies of the brain were made in eight patients who later were shown to have this disease. By these means a diagnosis was established in two of the eight patients. In four of seven patients with herpesvirus encephalitis the initial serum showed titers of complement-requiring neutralizing antibodies to Herpesvirus hominis that were significantly greater (4 or more) than the titers of non-complement-requiring neutralizing antibodies. In another patient the CRN/N was 4 on the 15th day in the hospital. Complement dependence generally disappeared in later sera. The CRN/N was 4 or more during two of four episodes of recurrent mucocutaneous herpes simplex and in five of 17 persons without current recognized herpetic infection. Disparities in times of rise and fall in complement-fixing, complement-requiring and non-complement-requiring neutralizing antibodies indicate that the molecular species of the three immunoglobulins are distinct.