An ELISA was established for the measurement of IgG antibody in human serum to the 30,000-Da native antigen of Mycobacterium tuberculosis and evaluated for its utility in the diagnosis of tuberculosis at the Instituto Nacional de Enfermedades Respiratorias in Mexico City. The test had a sensitivity of 70% in patients with sputum-positive active pulmonary tuberculosis and a specificity in control subjects of 100%. The accuracy of positive prediction was 100% and of negative prediction 93% for patients with pulmonary tuberculosis at this institute. Less favorable test characteristics were obtained for patients with miliary and pleural tuberculosis, in which the test had sensitivities of 22% and 14%. These results offer the promise of an accurate serodiagnostic test for pulmonary tuberculosis using readily obtained reagents.