Six Welsh Mountain pony foals were experimentally infected with a subtype 2 isolate of Equid Herpesvirus 1 (EHV-1) and subsequently examined for T cell mediated cytotoxicity against both subtypes. Cytotoxicity was not observed at 3 or 7 days after primary exposure but virus-specific, and genetically restricted, cytotoxicity of EHV-1-labelled autologous skin fibroblasts could be demonstrated 7 and 21 days after the animals were given a second exposure to live virus. Killing of subtype 2 antigen-labelled targets was more efficient than subtype 1 coated cells. This finding was paralleled by the observation that virus-neutralizing and complement-fixing antibody levels were subtype specific after the primary infection but after secondary exposure were directed against both subtypes. During primary infection the lymphocyte proliferative response to EHV-1 subtype 2 was not evident at 7 days post infection (dpi) but by 18 dpi was present in all animals. The second exposure produced an earlier (3 dpi) and larger proliferative response which was specific to the infecting isolate. The non-specific proliferative response to Concanavalin A mitogen indicated that virus infection induced a state of activation in circulating lymphocytes.