Mycobacterium tuberculosis is a facultative intracellular pathogen that has evolved the ability to survive and multiply within human macrophages. It is not clear how M. tuberculosis avoids the destructive action of macrophages, but this ability is fundamental in the pathogenicity of tuberculosis. A gene previously identified in M. tuberculosis, designated eis, was found to enhance intracellular survival of Mycobacterium smegmatis in the human macrophage-like cell line U-937 (J. Wei et al., J. Bacteriol. 182:377-384, 2000). When eis was introduced into M. smegmatis on a multicopy vector, sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the appearance of a unique 42-kDa protein band corresponding to the predicted molecular weight of the eis gene product. This band was electroeluted from the gel with a purity of >90% and subjected to N-terminal amino acid sequencing, which demonstrated that the 42-kDa band was indeed the protein product of eis. The Eis protein produced by M. tuberculosis H37Ra had an identical N-terminal amino acid sequence. A synthetic polypeptide corresponding to a carboxyl-terminal region of the deduced eis protein sequence was used to generate affinity-purified rabbit polyclonal antibodies that reacted with the 42-kDa protein in Western blot analysis. Hydropathy profile analysis showed the Eis protein to be predominantly hydrophilic with a potential hydrophobic amino terminus. Phase separation of M. tuberculosis H37Ra lysates by the nonionic detergent Triton X-114 revealed the Eis protein in both the aqueous and detergent phases. After fractionation of M. tuberculosis by differential centrifugation, Eis protein appeared mainly in the cytoplasmic fraction but also in the membrane, cell wall, and culture supernatant fractions as well. Forty percent of the sera from pulmonary tuberculosis patients tested for anti-Eis antibody gave positive reactions in Western blot analysis. Although the function of Eis remains unknown, evidence presented here suggests it associates with the cell surface and is released into the culture medium. It is produced during human tuberculosis infection and therefore may be an important M. tuberculosis immunogen.