Acute HIV infection is associated with a vigorous immune response characterized by the proliferation of selected T cell receptor V beta (BV)-expressing CD8(+) T cells. These 'expansions', which are commonly detected in the peripheral blood, can persist during chronic HIV infection and may result in the dominance of particular clones. Such clonal populations are most consistent with antigen-driven expansions of CD8(+) T cells. However, due to the difficulties in studying antigen-specific T cells in vivo, it has been hard to prove that oligoclonal BV expansions are actually HIV specific. The use of tetrameric major histocompatibility complex-peptide complexes has recently enabled direct visualization of antigen-specific T cells ex vivo but has not provided information on their clonal composition. We have now made use of these tetrameric complexes in conjunction with anti-BV chain-specific monoclonal antibodies and analysis of cytotoxic T lymphocyte lines/clones to show that chronically clonally expanded CD8(+) T cells are HIV specific in vivo.