The objectives of this study were to: 1) develop a technique to analyze the <i>in vitro</i> cytotoxic activity of lymphocytes from adult horses against equine herpes virus-1 (EHV-1) infected allogenic equine dermal fibroblasts (EDF); 2) evaluate the ability of a 72 hour in vitro incubation with interleukin-2 (I L-2) to enhance the lymphocytic cytolytic activity against EHV-1 infected EDF; 3) compare the cytotoxic activity among lymphocytes isolated from pregnant mares and non-pregnant mares against EHV-1 infected EDF; 4) ascertain if any correlations existed between the percent cytotoxicity and percentage of lymphocytes phenotypically identified by five different mouse-anti-equine monoclonal antibodies; and 5) determine if any correlation existed between virus-neutralizing antibody titers and the percent cytotoxicity. Results of the study indicate that <i>in vitro</i> cytotoxic activity of equine lymphocytes against EHV-1 infected allogenic fibroblasts can be measured with a standard 4 hour 51Cr release assay. This activity was enhanced by an <i>in vitro</i> incubation with IL-2. The cytolytic activity of freshly isolated lymphocytes was greater for non-pregnant than pregnant mares. However, after IL-2 stimulation the cytolytic activity was greater for lymphocytes from pregnant mares. A positive correlation was not detected between the percentage of phenotypically identified cells and the percent cytoxicity, although several negative correlations were present. This suggests that the cytotoxic activity was either not mediated by any of the phenotypically identified cell populations or that the activity was mediated by several different cell populations. No correlation was detected between virus neutralizing antibody titers and the percent cytotoxicity. / Master of Science