A whole-blood lymphocyte stimulation test was used in the diagnosis of 200 cases of tularemia. Ninety-one of the patients involved had ulceroglandular infection, 81 had pulmonary infection, 13 had glandular infection, four had oropharyngeal infection, and one had oculoglandular infection. The clinical picture was obscure in the remaining 10 cases. Results of the lymphocyte stimulation test became positive in 21.3% of cases during the first week, when an agglutinating titer of greater than or equal to 1:80 was found in 2.1% of patients. During the second week, these proportions were 96.8% and 53.3%, respectively. In only one patient with tularemia (0.5%) did the stimulation test result remain negative, and only one (0.5%) of the 211 control patients had a positive result in the first test. The development of cell-mediated immunity followed the same pattern in patients with ulceroglandular tularemia as in those with pulmonary tularemia.