This report presents data showing that several virus isolates recovered in Argentina, mainly from calves with non-purulent meningo-encephalitis, represent a hitherto unrecognized antigenic variant of BHV-1. The following experimental approaches have been adopted to demonstrate both the unique features among and the relatedness with BHV-1 of these isolates: i) crossed serum neutralization test with rabbit immune sera, ii) analysis by SDS-polyacrylamide gel electrophoresis of radio-labeled virus induced polypeptides and glycoproteins, iii) discriminating reactivity of a panel of monoclonal antibodies which recognize known virus types, and iv) restriction endonuclease analysis of viral DNA. Another strain of BHV-1, which exhibits a specific neuropathogenic potential [Hall et al., Austral. Vet. J. 42, 229-237 (1966)] shares all major features with the viral strains originating from Argentina. Our results imply that antigenic variants of BHV-1 exist and that they can be accurately and easily identified and differentiated by the available methods.