Cytotoxic lymphocytes against infectious bovine rhinotracheitis virus (IBRV)-infected cells were induced by in vitro stimulation with IBRV antigen of peripheral blood leukocytes obtained from hyperimmune cattle. After one in vitro stimulation with IBRV, cytotoxicity was detected against IBRV-infected autologous target cells, but not against heterologous IBRV-infected or K562 target cells. After 4 in vitro IBRV stimulations at weekly intervals (in the presence of autologous feeder cells followed by interleukin 2 treatment at 2-day intervals), cytotoxicity was detected against autologous-, and to a lesser extent, heterologous-infected cells. Most cells in cytotoxic cultures were positive to a monoclonal antibody shown to react with bovine T lymphocytes.