 As part of on-going studies conducted in the Orizaba Health Jurisdiction in southeastern Mexico, household contacts of pulmonary TB patients who recently converted their tuberculin test and TSTve contacts from randomly selected control households with no history of TB within the last 2 years remote contacts will be enrolled. We assume that these individuals are infected with Mycobacterium tuberculosis. Additional confidence that all subjects enrolled are latently infected will come from ELISPOT analysis of the response to ESAT-6 and CFP-10. We propose to administer INH to 100 TSTve recent and 100 TSTve remote contacts for 6 months. To control for spontaneous fluctuations of biomarker levels, we propose to defer therapy by 3 months to half the subjects in each group. Thus, four groups will be defined: 