Little is known about T cell antigens involved in immunity against Mycobacterium tuberculosis. Most model systems use in vitro culture of human T lymphocytes with bacterial lysates or secreted proteins as antigens. In this study, proteins from 3-week-old M. tuberculosis culture filtrates were separated by two-dimensional PAGE and subsequently transferred into soluble phase. The resulting 480 fractions were screened with T lymphocytes from tuberculosis patients and healthy contacts. T cells from all 9 patients and from 8 of 10 tuberculin-positive contacts preferentially responded to a cluster of acidic proteins (pI 4-5) with molecular masses of 30-100 kDa, although they also recognized a number of other fractions. In contrast, of 7 tuberculin-negative contacts, 4 were not and 3 were only weakly stimulated by this cluster region. Therefore, this distinct cluster of secreted proteins seems to comprise dominant T cell antigens of M. tuberculosis.