We have used serological proteome analysis in conjunction with tandem mass spectrometry to identify and sequence a novel protein, Mtb81, which may be useful for the diagnosis of tuberculosis (TB), especially for patients coinfected with human immunodeficiency virus (HIV). Recombinant Mtb81 was tested by an enzymelinked immunosorbent assay to detect antibodies in 25 of 27 TB patients (92%) seropositive for HIV as well as in 38 of 67 individuals (57%) who were TB positive alone. No reactivity was observed in 11 of 11 individuals (100%) who were HIV seropositive alone. In addition, neither sera from purified protein derivative (PPD)-negative (0 of 29) nor sera from healthy (0 of 45) blood donors tested positive with Mtb81. Only 2 of 57 of PPD-positive individuals tested positive with Mtb81. Sera from individuals with smear-positive TB and seropositive for HIV but who had tested negative for TB in the 38-kDa antigen immunodiagnostic assay were also tested for reactivity against Mtb81, as were sera from individuals with lung cancer and pneumonia. Mtb81 reacted with 26 of 37 HIV ؉ TB ؉ sera (70%) in this group, compared to 2 of 37 (5%) that reacted with the 38-kDa antigen. Together, these results demonstrate that Mtb81 may be a promising complementary antigen for the serodiagnosis of TB. The 38-kDa antigen was expressed in E. coli by using a hexahistidine tag similar to that used for Mtb81 (see below). The TB lysate was prepared by alternately homogenizing and sonicating three 100-mg ampoules of dessicated M. tuberculosis H37Ru (Difco, Franklin Lakes, N.J.) three times in 25 ml of 10 mM Tris (pH 8) containing 2% Nonidet P-40. The mixture was spun for 10 min at 13,000 ϫ g. The supernatant was used as the TB lysate.Study population. Blood was drawn from all patients after informed consent was obtained. Serum samples were obtained from individuals who had pulmonary TB alone prior to treatment (culture and/or acid-fast bacillus smear positive; see Results) or who had documented coinfections with HIV, as evidenced by a positive HIV type 1 and 2 antibody ELISA. These included samples from Uganda and South Africa that were obtained from Milton Tam (Program for Appropriate Technology in Health, Seattle, Wash.). Additional HIV ϩ TB ϩ serum samples from South Africa and samples from patients with lung cancer and pneumonia (China) were obtained from Robert Cole (AMRAD-ICT, French's Forest, Australia). This second group of HIV ϩ TB ϩ samples was selected on the basis of their negativity for the 38-kDa antigen in the AMRAD-ICT rapid test but smear-and/or culture-positive results (6, 34). To further evaluate the specificity of the Mtb81 antigen, we obtained sera from individuals who were PPD positive (Ͼ10-mm reaction zone) (culture, clinically, and radiographically negative for TB) and PPD negative (King County Tuberculosis Clinic, Seattle, Wash.). Samples from healthy blood donors (United States) were obtained from Boston Biomedica (West Bridgewater, Mass.), and sera from individuals who were HIV seropositive alone were obtained fr...