The invention relates to novel secreted antigens from mycobacteria capable of evoking early (within 4 days) immunological responses from T-helper cells in the form of gamma-interferon release in memory immune animals after rechallenge infection with mycobacteria of the tuberculosis complex. The antigens are present in short term filtrates (ST-CF) from cultured mycobacteria belonging to the tuberculosis complex. One of these antigens, a polypeptide with an apparent molecular weight of 6 kDa, has been identified, and the DNA encoding the polypeptide has been cloned and sequenced. The antigens of the invention are believed useful especially in vaccines, but also in diagnostic compositions, especially for diagnosing infection with virulent mycobacteria. Also disclosed are nucleic acid fragments encoding the antigens as well as methods of immunizing animals/humans and methods of diagnosing tuberculosis.