The behavior of leukocytes, erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) in tularemia caused by Type B of Francisella tularensis was analyzed in different clinical forms and severities of disease in 101 adult tularemia patients. The mean leukocyte count was 8.3 X 10(9)/l and the leukocyte differential count was also usually normal. The behavior of leukocytes was similar in different clinical forms and severities of tularemia. The changes in differential counts were milder than reported earlier. Both CRP and ESR were higher in severe than in milder forms of tularemia (p less than 0.05 and p less than 0.01, respectively), and CRP was higher in pulmonary than in ulceroglandular tularemia during the second week (p less than 0.05). There were high individual CRP concentrations of up to 225 mg/l in acute tularemia but there were also low CRP values (10-40 mg/l). Thus the behavior of CRP in tularemia resembled that seen in tuberculosis, and CRP did not always allow differentiation of tularemia from viral diseases.