Jembrana disease is an acute and severe disease of Bali cattle (Bos javanicus) endemic in Indonesia that is caused by a bovine lentivirus designated Jembrana disease virus (JDV). Previous studies have demonstrated that it is possible to induce a protective immunity against the disease by immunisation with a crude whole virus vaccine prepared from the tissues of infected cattle. This vaccine has been demonstrated to ameliorate the clinical signs of disease resulting from exposure to virus infection but a safer vaccine amenable to commercial production techniques is required. JDV, like all lentiviruses, encodes a transcriptional trans-activator Tat protein that is encoded from one or both of two exons of the tat gene. Tat is particularly essential for virus replication and it was hypothesised that the induction of an immune response in cattle against JDV Tat may effect protection against virus infection. Investigations were therefore conducted on JDV Tat to provide basic information on the protein that would enable it to be further investigated as a potential immunogen for incorporation into vaccines for the control of Jembrana disease. Analysis of tat transcripts obtained from tissues of cattle infected with three strains of JDV suggested that, during the acute clinical disease, Tat produced at this stage of the disease process was translated from the first coding exon only. Nucleotide variation in this exon, which would have translated into amino acid variations in the Tat protein, was evident especially between strains from geographically different regions of Indonesia. There was; however, conservation of the essential functional domains of cysteine-rich, core and basic regions, which suggested immunity to a single Tat protein might protect against infection by heterologous strains. Subsequent studies on Tat reported in the thesis therefore concentrated on the protein encoded by tat exon 1 of a single strain of JDV. The exon 1 of tat was cloned into the pGEX vector and recombinant Tat expressed in Escherichia coli. Methods for the purification of the expressed protein were developed. Immunogenicity of the recombinant protein was initially demonstrated by inoculation of the protein into a sheep which developed a high titred specific antibody response. Antibodies induced by this recombinant protein recognised native Tat proteins produced by three JDV strains in Bali cattle and provided a valuable reagent for the subsequent detection of Tat in vitro and in vivo. Aspects of the antibody response to Tat were determined in cattle that had been infected naturally or experimentally with JDV, and compared with the levels of antibody to the immunodominant capsid protein. Tat antibodies were detected in 23 % of 128 Bali cattle from Jembrana disease-endemic areas of Indonesia; in all these cattle, evidence of previous virus infection had been demonstrated by detection of antibody to the JDV capsid protein by Western blot analysis. In cattle experimentally infected with JDV, low levels of serum antibody to Tat were detected by Western blot in the first month post-infection but the levels of antibody then decreased; levels of antibody to the JDV capsid protein increased over the 6-month observation period following infection. The detection of Tatantibody soon after the acute clinical disease suggested that this protein is secreted extracellularly during JDV infection in cattle. In contrast to the antibody response to Tat in JDV-infected cattle, an apparently greater antibody response to Tat was induced by injection of recombinant Tat in Bali cattle. The strong antibody response resulting from inoculation of the recombinant Tat and low levels of Tat antibody in animals that had been naturally or experimentally infected with virus suggested there might be a conformational difference in the recombinant and native Tat protein and that the native protein was a poor immunogen, or that the levels of Tat in infected cattle were too low to induce a strong antibody response. As an alternative means of inducing an immune response to JDV Tat, perhaps one associated with a greater cell-mediated rather than an antibody response, a candidate tat DNA vaccine was produced by insertion of tat exon 1 into a DNA vaccine vector. Transfection of this naked DNA plasmid into mammalian cells induced the expression of a functional Tat protein which maintained antigenicity. The results suggested this construct merits further animal studies attempting to induce a protective immune response against Jembrana disease in cattle. A method of assaying the trans-acting function of Tat was also developed which will have application for quality control procedures for large-scale production of tat DNA vaccine.