Fludarabine and alemtuzumab (humanized anti-CD52 antibody, Campath®) are both used as routine therapy for patients with B-CLL. Fludarabine and in particular alemtuzumab, in addition to their antitumor effects, induce long-lasting suppression of T-cell numbers in blood. T-cell suppression, in combination with advanced disease, may result in an increased risk of opportunistic and other infections. In addition to a decrease in circulating T cells, functional defects in T cells have been described following alemtuzumab therapy for non-malignant disorders; however, only a limited amount of information exists about the effect of alemtuzumab on T-cell function in patients with B-CLL. The aim of the present study was to characterize in detail various aspects of T-cell function in patients with B-CLL who were in stable unmaintained PR or CR following fludarabine (n = 9) and alemtuzumab (n = 10) treatment as well as in age-matched healthy control donors (n = 10). CD4 and CD8 T-cell subsets of freshly obtained peripheral blood mononuclear cells (PBMC) were analyzed by flow cytometry to measure the expression of TCR-CD3 ζ chain, p56Lck, p59Fyn, ZAP-70, and PI3 Kinase as well as intracellular production of IFN-γ and IL-4. Additional analyses were performed to measure the proliferative response capability to a recall antigen (PPD) in alemtuzumab-treated patients and (as a control) indolent, untreated B-CLL patients (n = 11). The expression of IFN-γ and IL-4 (measured as mean fluorescence intensity [MFI]), but not the total number of positive cells, was significantly higher in CD4 and CD8 T cells for both CLL groups compared to healthy donors with the highest expression observed in alemtuzumab-treated patients. The total number of T cells that expressed the five signaling molecules was significantly lower in treated patients versus control donors; however, there were no differences between fludarabine- and alemtuzumab-treated patients. MFI analysis showed a significant increase in the TCR-CD3 ζ chain in fludarabine-treated patients compared to control donors; however, MFI analysis revealed no other significant differences between control group patients and fludarabine- and alemtuzumab-treated patients. Moreover, the proliferative response to PPD was not significantly different in alemtuzumab-treated patients and indolent CLL patients. In conclusion, these results suggest that the intrinsic signal transduction machinery in T cells is relatively well-preserved, such that T cells remain functionally intact following fludarabine or alemtuzumab therapy despite a marked reduction in their numbers. These results provide a better understanding of the immune suppression induced by alemtuzumab and fludarabine therapy in B-CLL patients.