The perchloric acid soluble, heat stable, and ethanol insoluble antigen of M. tuberculosis (TB-PBE) was prepared, and antigenicity of this antigen was studied in vivo and in vitro. TB-PBE showed a single band of 60 kDa by SDS-PAGE. Sera from the patients with active pulmonary tuberculosis did not react with this antigen by ELISA. A delayed hypersensitivity skin reaction was induced with this antigen and was correlated with the reaction with PPD. Skin biopsy was performed in this skin lesion induced by TB-PBE and stained by H-E and immunohistochemical methods. TB-PBE induced an inflammatory lesion similar to a lesion induced by PPD. Blastogenic activity of the peripheral blood mononuclear cells stimulated by TB-PBE increased, and showed a peak reaction at 7 days after stimulation. The blastogenic activity changed in a dose-dependent manner. After stimulation with TB-PBE, mononuclear cells were analyzed by FACS. DR+ T cells and CD4/CD8 ratio increased after stimulation by TB-PBE. These cells secreted IL-2, not IL-4 after stimulation with TB-PBE. In the immunofluorescence test, mouse antiserum against TB-PBE showed a positive reaction with M. tuberculosis and showed cross-reactivity with M. bovis and other atypical mycobacteria, but not with S. aureus. With these results, it is evident that TB-PBE is an antigen which can induce cell mediated immunity in vivo and in vitro.