The antigenic relationships among 50 strains of equine herpesvirus (EHV) were studied by neutralization tests using antisera prepared in rabbits against four EHV reference strains: types 2 and 3, cytomegalo-like virus 82-A, and our leukocyte isolant H-40. No distinctive antigenic differences among reference strains were demonstrated in reciprocal neutralization tests but each antiserum neutralized its homologous virus more rapidly than any heterologous strain. Forty-six EHV strains isolated from peripheral blood leukocytes of apparently healthy horses were antigenically indistinguishable from each other and from the four reference strains. Their high degree of antigenic relatedness suggests that these viruses are isolants of a single, widely distributed serotype of which type 2 (LK) strain is a typical representative.