Most human peripheral blood gammadelta T lymphocytes respond to hitherto unidentified mycobacterial antigens. Four ligands from Mycobacterium tuberculosis strain H37Rv that stimulated proliferation of a major human gammadelta T cell subset were isolated and partially characterized. One of these ligands, TUBag4, is a 5’ triphosphorylated thymidine-containing compound, to which the three other stimulatory molecules are structurally related. These findings support the hypothesis that some gammadelta T cells recognize nonpeptidic ligands.