THE majority of T cells bear the T-cell receptor (TCR) αβ complex which recognizes foreign antigen peptides only in the context of self major histocompatibility complex (MHC) molecules¹. Such T cells function in a variety of effector roles and secrete cytokines that mediate the activation and differentiation of other cells in the immune system. Recently, a small subpopulation T cells was found to bear a distinct TCR composed of γ and δ subunits². In man, TCR γδ⁺ cells are distributed as ~5 per cent of the CD3⁺ cells in all organized lymphoid organs as well as in the skin- and gut-associated lymphoid tissues³. Although a limited number of germ-line genes encode the TCR γ and δ subunits, extensive junctional variation particularly in the δ gene, results in unprecedented diversity for this receptor^4,5 The nature of the specificity and immunological functions of these T cells remains enigmatic. We report here that in contrast to the normal low frequency of γδ-bearing cells in lymphoid tissues, peripheral blood, or normal skin, the frequency is increased five to eightfold in particular granulomatous reactions of leprosy. TCR γδ⁺lymphocyte lines from these leprosy skin lesions proliferate in vitro specifically to mycobacterial antigens. This reactivity to foreign antigens appears to require presentation in the context of self-molecules. Moreover, culture supernatants from activated γδT lymphocytes induce adhesion and aggregation of bone-marrow monocytes in the presence of granulocyte monocyte-colony stimulating factor (CSF), suggesting that products of γδ-bearing T cells may play a role in the immune response, possibly by stimulating granuloma formation.