NK cell receptors for MHC class I molecules (MHC-NKR) can be expressed by T cell subsets. The restricted repertoire and phenotypic characteristics of MHC-NKR + T cells indicate that expression of MHC-NKR is acquired upon antigenic challenge and might promote expansion of T cells. Previous studies performed on in vitro generated ab T cell clones concluded that MHC-NKR expression was not a clonal attribute. Here, we examined a massive monoclonal expansion of a non-leukemic cd T cell population found in the peripheral blood of a lung-transplanted patient who suffered from a cytomegalovirus infection. Despite their monoclonality, these T cells displayed a heterogeneous and stable in vivo Ig-and lectin-like MHC-NKR phenotype. Twenty percent of the cells displayed a CD94 + NKG2A + phenotype, and 10% were labeled with an anti-CD158b1/b2/j monoclonal antibody. A CD158b/j + cd T cell clone derived in vitro from patient's peripheral blood lymphocytes was shown to express the activating form CD158j (KIR2DS2), which once cross-linked stimulated the clone cytolytic function and costimulated the TCR-induced production of cytokines, independently of the killer-activating receptor-associated protein (KARAP). In conclusion, heterogeneity of MHC-NKR expression confers a functional intraclonal diversity that may participate to induction of specific cd T cell effector functions or proliferation upon pathogen challenge.