The immune response to M. tuberculosis engages T-cell medicated reactions which determine the degree of resistance and also the clinical pattern of disease. Although antibodies produced by the infected host have yet no proven protection, they are appilicable for alternative diagnostic tests in tuberculosis. Preparation of purified antigens from Mycobacteria with specific antigenic determinants would improve serological diagnosis in tuberculosis. The antigen was prepared by extraction of M. tuberculosis with perchloric acid which was found to be major bands of 71, 42, 38 and 10 kD by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Thirteen sera from patients with active tuberculosis showed positive reactions to TB-P by ELISA, but the reaction to perchloric acid extract antigen is weaker than to PPD. The IgG subclass profiles to TB-P were IgG1, IgG2 and IgG4. By means of western blotting, the antibodies in the tuberculous patients showed the reaction with antigens of lower molecular weight such as 14, 12, 11 and 10 kD. These results suggested that the perchloric acid soluble antigen of M. tuberculosis might be more related with cell-mediated immune reactions rather than humoral immune reactions.