The aim of the present study was to determine the cellular immune response on the course of brucellosis by investigating the proliferation response of T-cell subsets to phytohemagglutinin (PHA), that is, nonspecific rnitogen in the patients with acute and chronic brucellosis. The study was performed in 19 patients with untreated brucellosis (acute, n = 11; chronic, n = 8) and 19 healthy controls. Standard tube agglutination and Coombs tests for brucellosis were performed. CD4+ and CD8+ T cell were investigated by the flow cytometry and sorting methods in all of cases. After these cells were cultured and stimulated with PHA, [H-3]-thymidine uptake and stimulation indices (SIs) were established. In all of the patients with brucellosis, CD4+ SIs and CD8+ SIs were found to be 1.40 0.63 and 1.45 + 0.42, respectively, and in the controls CD4+ SIs and CD8+ Sls were 1.59 +/- 0.36 and 1.64 +/- 0.37, respectively. In acute cases, CD4+ Sls were 1.71 +/- 0.64 and CD8+ Sis were 1.54 +/- 0.45. CD4+ SIs were 0.97 +/- 0.25 and CD8+ Sls were 1.32 +/- 0.37 in chronic cases. Although in acute cases CD4+ SIs and CD8+ SIs were not different from those in the control group, CD4+ SIs of chronic brucellosis cases were found to be significantly low as compared with those of acute brucellosis cases and the controls (P < 0.01). CD8+ SIs of acute and chronic brucellosis cases were not found to be different from those in the controls. Brucella agglutination titers of the patients with acute and chronic brucellosis were not found related with CD4 SIs and CD8 Sls. The findings of significantly low results of CD4+ T-cell proliferative responses of chronic brucellosis to PHA as compared with control and acute brucellosis cases remind that the development of chronic infection might be a result of T-helper proliferation defect. (c) 2005 Elsevier Inc. All rights reserved.