Genotoxic DNA damage due to inhalation anaesthesia has been demonstrated in human lymphocytes. In order to evaluate anaesthesia-associated changes in cell-mediated immunity on the basis of a potential DNA damage as a health risk in horses, single cell gel electrophoresis and lymphocyte proliferation assay were performed on equine lymphocytes which were obtained before, during and after regular castration under inhalation anaesthetic. No significant lymphocytic DNA damage due to isoflurane anaesthesia was observed, whereas lymphocyte proliferative reactivity and lymphocyte counts decreased significantly (p≤0.05) during and after anaesthesia. The present study thus indicates that the combined anaesthesia does not result in significant DNA damage, which hence cannot be held responsible for the observed changes in the immune response of equine lymphocytes. However, the recognized compromises of immune function ought to be considered especially in immunologically challenged animals.