Jembrana disease is a severe and acute clinical disease in Bali (Bos javanicus) cattle with a case fatality rate of about 20%, and a mild sometimes subclinical disease in other cattle types and buffalo. The aetiological agent has been identified as a lentivirus, designated as Jembrana disease virus (JDV). Preliminary sequence analysis has confirmed the identity of JDV as a lentivirus and has shown that it is distinguishable from BIV. There is antigenic cross-reactivity between the capsid protein of JDV and the previously identified bovine lentivirus designated bovine immunodeficiency virus (BIV). Serological tests that detect antibody to the capsid protein of JDV or BIV would not differentiate between antibody due to infection by either virus. The diseases induced by BIV and JDV infection in cattle are very different, and the pathogenesis of JDV infection in Bali cattle is unusual for a lentivirus infection.