The purpose of this project is to evaluate the effect of two accepted regimens for treating latent tuberculosis infection (LTBI) in jail. Tuberculosis (TB) in incarcerated populations continues to be a serious problem, due to the large proportion of persons who are at high risk of both having latent tuberculosis infection (LTBI) and developing active disease. Completion of treatment of LTBI, while an important component of overall TB control efforts, has not been successful in jails. This is primarily because inmates are frequently released before finishing a 6-9 month course of standard therapy, and have low rates of completing therapy in the community. This study proposes to look at toxicity and adherence for this 4-month regimen compared to the nine-month regimen of to isoniazid (INH), and to examine costs, both cost of delivered care and the cost of TB disease prevented, with examination of reasons for completion or noncompletion of therapy. Short-course therapies for LTBI may address this problem but they are more expensive and have not been studied adequately to answer questions about side effects, completion rates, and overall cost. The investigators propose a randomized trial to test the effects of a short course therapy, rifampin (600 mg orally) given daily for 4 months, as compared to (INH) (900 mg orally) given twice weekly for 9 months. Both regimens are listed by the Centers for Disease Control and Prevention (CDC) and the American Thoracic Society as acceptable treatments for persons with LTBI. The study participants will include 972 San Francisco Jail inmates, 18 years and older, enrolled over a 28-month period, for a sample of 486 in each study group. Subjects, followed in jail and after release, will be followed to test three hypotheses: the null hypothesis of a difference in toxicity of rifampin as compared to INH within a 95% confidence interval of (.4-1.87) and no difference by study group in adherence and in cost-effectiveness. A secondary aim is to describe reasons for completion or noncompletion of therapy. Toxicity is defined as complications leading to stopping drug. Adherence is defined as completion of care, or 120 doses taken within 6 months for the rifampin group and 76 doses of INH taken within 12 months for the INH group. Cost effectiveness will be calculated as the total cost of care (nursing, medical, laboratory, as well as facility costs), and measured against costs of TB cases prevented. All treatments will be administered by directly observed therapy (DOT) in jail, and by DOT after release at the San Francisco TB Clinic. Counseling on adherence (going to the TB Clinic if released before completing therapy) and on possible side effects will be given to all study subjects at enrollment and during follow-up clinic visits. All subjects will be routinely evaluated by study personnel every two weeks for the first 6 weeks, and thereafter to detect possible drug toxicity including hepatitis, peripheral neuropathy, arthralgias, rash, memory loss, and other clinical symptoms. All patients will undergo laboratory assessment at regular intervals according to a schedule which compares study group participation and the usual care in the jail. All blood test results, and new symptoms or changes in symptoms found at follow-up, will be added to the jail medical record. A final interview will be done with subjects at the time that they have completed or not completed this course of therapy for LTBI, to determine reasons (barriers and enablers). Follow-up will continue for each subject for five years after enrollment into the study, to measure study endpoint (completion of care, taken off drugs for toxicity or loss to follow-up) and to measure subsequent treatment for LTBI or development of active TB by record review.