The complete sequencing of the Mycobacterium tuberculosis genome offers a unique opportunity to fully elucidate the biology of this human pathogen. One aspect of significant importance is the definition of T cell antigens. This report describes the development and implementation of a proteomic approach to defining such antigens. Large quantities of subcellular protein fractions of M. tuberculosis were resolved by two-dimensional liquid phase electrophoresis (2-D LPE), resulting in 355 and 299 fractions of culture filtrate and cytosolic proteins, respectively. Analysis of these fractions against splenocytes of C57Bl/6 mice infected with M. tuberculosis resulted in the identification of 37 fractions that stimulated a dominant T cell response, as measured by the production of interferon-gamma. Additionally, when the 2-D LPE fractions were assayed against splenocytes harvested at 10 and 40 days post infection significant changes in the T cell response were observed. Molecular characterization of the proteins contained in each of the 38 immunodominant fractions by liquid chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry resulted in the identification of 30 individual proteins. Many of these represented previously defined antigens. However 17 of these proteins were novel T cell antigens. The data presented demonstrate that proteomics offers a rapid and facile approach for elucidation of immunodominant T cell antigens of pathogenic bacteria.