Human T-cell chronic lymphocytic leukemia (T-cell CLL) is a heterogeneous disease characterized by a monoclonal malignant proliferation of T cells in which the T-cell receptors (TCRs) can be, when expressed, considered to be membrane tumor-specific antigens. Owing to the increasing number of available monoclonal antihuman TCR reagents, it could be of interest to evaluate the feasibility of anti- TCR treatment during T-cell CLL. To test the therapeutic potentiality of anti-TCR monoclonal antibodies, we first analyzed the intraclonal variability in two terminally ill patients suffering from TCR alpha beta-positive cell CLL bearing different immunophenotypes. The cDNA corresponding to the variable regions of the TCR beta chains originating from the malignant T cells were amplified, cloned into M13 phages, and sequenced. The sequence analysis of multiple independent clones showed no intraclonal variability, with no evidence for ongoing hypermutation in the V beta region genes. The relevance of these findings with regard to an anti-V beta therapy and the comparison with similar analysis during B-cell monoclonal lymphoproliferations are discussed.