31 subjects with tularemia recently or up to 11 years earlier were studied for cell-mediated immunity against Francisella tularensis using formalin-killed bacteria as antigen in the lymphocyte blast transformation test. Lymphocytes from all the subjects responded to F. tularensis antigen both in separated mononuclear cell and whole blood cultures, whereas lymphocytes from 12 controls responded not at all or only weakly to hig antigen concentrations and only in separated mononuclear cell cultures. The strength of the response remained on the same level as in the cases of recent infection up to 11 years. There was no correlation between the lymphocyte responses and the serum antibodies agglutinating F.l tularensis antigen. Purified protein derivative of tuberculin equally stimulated the cells from the tularemia and control subjects. The lymphocyte stimulation methods can bae used to diagnose infections caused by F. tularensis and to measure cell-mediated immunity and resistance against such infections.