Herpes simplex virus types land 2 specific immune sera, sera from convalescent and healthy humans, and bovine herpes mammillitis virus-specific hyperimmune sera showed cross-neutralization with the heterologous viruses. The antigenic relationship of these herpes viruses was confirmed with complement fixation, immunodiffusion and immunofluorescence tests. Bovine herpes mammillitis infection of test animals protected them against a fatal herpes simplex virus infection. The DNA of the bovine herpes mammillitis virus (63% guanadine + cytosine; G + C) shared about 14% base sequence homology with that of herpes simplex virus type 1 (66% G + C), whereas the genetic relatedness with the infectious bovine rhinotracheitis and pseudorabies virus was less than 8%.