Nine monoclonal antibodies were raised against purified protein derivative (PPD) of tuberculin in mice previously treated with Bacilli Calmette Guérin (BCG). The antibodies also reacted with a culture filtrate from Mycobacterium tuberculosis strain H37Rv. In immunoblotting after SDS-PAGE the reaction with PPD was seen as a diffuse smear, whereas ammonium sulphate-precipitated proteins from H37Rv gave well-defined bands ranging from 10 to 65 kDa. Enzyme immunoassay showed that both PPD and H37Rv antigens were able to inhibit binding of the antibodies to PPD coated microtitre wells, suggesting that the antibodies reacted with continuous epitopes. A 12 kDa protein purified by immunoaffinity chromatography from H37Rv antigens was tested intradermally in M. tuberculosis MNC3 sensitized guinea pigs and gave a delayed type hypersensitivity reaction.