A major step toward the eradication of tuberculosis in the United States has been the use of isoniazid for chemoprophylaxis in certain persons who have positive tuberculin skin tests but no other evidence of active infection. Chemical trials have demonstrated the effectiveness of chemoprophylaxis in groups where there is a relatively high risk of active tuberculosis. However, only the practicing physician can identify and offer chemoprophylaxis to many other susceptible persons. Even if the patient is a candidate for isoniazid, the risk of developing tuberculosis must be weighed against the cost and possible adverse effects of the drug. If isoniazid is given, the physician must be alert to the signs of possible drug toxicity. If isoniazid is not given, he must anticipate the development of active tuberculosis in susceptible persons.