(1) Isolation of herpetic virus and immunological studies of the serum and aqueous humor are two complementary methods of investigation in the diagnosis of ocular herpes simplex infection, their relative value depending on the clinical situation. (2) Of the different immunological techniques that of antibody titration by the hemagglutination method is an easy, sensitive test, but only if the Goldmann-Witner ratio is calculated. By this technique, the diagnosis of herpetic infection can be made in unusual clinical presentations or old cases in which all specificity had disappeared.