When varying dilutions of human sera were mixed with constant amounts of the two types of herpesvirus, it was observed that either type 1 virus was neutralized at a greater serum dilution than type 2, type 2 was neutralized at a greater serum dilution than type 1, or the two types were neutralized equally well. The relative antibody titer of type 2 virus to type 1 virus was found to correlate with the location of herpetic lesions experienced by the patients. When the ratio was expressed as an index (II/I index), sera from persons with oral herpesvirus type 1 infections, for the most part, yielded values of less than 85 whereas sera from persons with genital herpesvirus type 2 infections yielded values of 85 or greater. The same relationship was demonstrated using either the plaque-reduction or the microneutralization test. The plaque-reduction test, microneutralization test and neutralization kinetics test were compared by analyzing a group of sera for herpesvirus type 2 antibodies by all three methods. There was agreement between the three methods in 83% to 89% of the determinations. Thus antibodies to the two herpesviruses can be quantitated in the same serum by neutralization techniques employing varying serum dilutions and a constant amount of virus, and the same antibody activity is measured as in the neutralization kinetics test.