This thesis describes the development, optimisation and use of assays to measure equine herpes virus-specific proliferative and cytotoxic Tlymphocyte (CTL) responses in the blood of horses. Equine T cell blast cells stimulated from peripheral blood mononuclear cells (PBMC) with pokeweed mitogen were found to perform best as targets for CTL in "Cr release assays. CTL induced in vitro with an abortigenic strain of EHV-1 (EHV-1/Ab4) were shown to be antigen specific, genetically restricted and predominantly of the CD4' CD8+ phenotype. Cross-reactive CTL were induced in vitro with live EHV-4 virus, which killed EHV-1 infected blast cells. A proportion of EHV-1 induced CTL were shown to be directed against the immediate early gene products. A proliferative LDA was used to determine whether the frequency of precursor T cells detected before challenge with EHV-1 correlated with immune status. The precursor frequency of antigen-specific T cells increased in 3 out of 4 horses after infection. However, there was no correlation between precursor frequency and outcome of infection. A LDA was developed and used to evaluate the precursor frequencies of EHV-1 and EHV-4 induced CTL after infection with these viruses. Pre-infection CTLp frequencies in susceptible animals were < 1/150,000. CTLp frequencies in animals which were immune to EHV-1 were between 1/10,000 and 1/20,000. To my knowledge this is the first report of the use of LDA techniques in the horse. The development and use of CTL LDA assays have provided new information on CTL responses in horses after EHV-1 and EHV-4 infection.