This investigation was undertaken to assess the effectiveness of an enzyme-linked immunosorbent assay (ELISA) using A60 antigen in ascertaining diagnosis in hospitalized patients suspected to have pulmonary tuberculosis (TB) but with negative sputum stains. Cultures were performed to confirm active or inactive disease. IgG and IgM antibody activity was determined by adding a 1:100 dilution of serum to plates coated with A60 antigen. After addition of peroxidase-conjugated antihuman IgG or IgM and color development, optical density (OD) was determined. A total of 83 patients was studied, taking into account their current disease status and prior history. Using as a cutoff value the mean value +/- 2 SD measured in the negative culture, no TB history group, that is, OD = 0.50 for IgG measurements and 0.43 for IgM measurements, the sensitivity, specificity, and positive predictive value of IgG measurements were equal to 48, 71, and 50%, respectively. Using IgM measurements, these parameters were equal to 76, 98, and 95%, respectively. Combining the results of IgG and IgM measurements, sensitivity, specificity, and positive predictive value were equal to 68, 100, and 100%, respectively. Thus, the ELISA described here can greatly facilitate the diagnosis of TB in patients with negative smears.