Two types of T lymphocytes can be discriminated on the basis of expression of either the classical T-cell receptor (TCR) alpha beta or the more recently identified TCR gamma delta. Whereas TCR alpha beta + lymphocytes are known to respond to recognition of antigen in the context of major histocompatibility complex molecules by proliferation, lymphokine secretion, and/or cytotoxicity, the potential ligand specificities and functions of TCR gamma delta + cells have not been completely unraveled. Antibodies specific for either receptor type are important tools to elucidate the role TCR gamma delta + cells play in the immune system. They can be used to quantify TCR gamma delta + cells and TCR alpha beta + cells in normal and disease states, to isolate both T-cell subsets, and to perform in vitro functional assays. Only few antibodies reactive with common determinants on either TCR alpha beta or TCR gamma delta are available. Generally, the monoclonal antibody (mAb) WT31 is used for definition of viable human TCR alpha beta + cells. However, WT31 has recently been shown to cross-react with TCR gamma delta. We describe an mAb, BMA031, that combines the unique features of reactivity with intact viable cells and true specificity for a common determinant on the TCR alpha beta/CD3 complex. Its performance in immunofluorescence staining and immunochemistry has been compared with that of WT31 and anti-TCR gamma delta mAbs, using TCR alpha beta and TCR gamma delta expressing cells isolated from blood and bone marrow of healthy individuals and immunodeficient patients.