nounced the discovery of a cure for tuberculosis. The cure consisted of giving patients subcutaneous doses of tuberculin, a brownish, transparent liquid obtained from culture filtrates of MWcobacterium tuberculosis. This treatment caused a febrile reaction within 4 or 5 hours. In most cases, the fever was accompanied by vomiting, rigors, and other constitutional symptoms. Koch reported that the administration of increasing daily doses of tuberculin resulted in the rapid healing of mild cases of tuberculosis and in slow, progressive improvement in more serious cases [1]. Patients who did not have tuberculosis experienced slight pains in the limbs and transient fatigue when injected with tuberculin. Less than a year after the announcement, Koch's tuberculin was disproved as a cure for consumption [1]. Koch did not realize that he had discovered what would become one of the most widely used diagnostic tests ever developed. The observation of different responses to tuberculin in patients with and without tuberculosis led to the development of new methods of administering the antigen. The use of these new methods eliminated systemic symptoms but caused a local reaction at the injection site. Various methods of administration were tested, including the Pirquet cutaneous test, the Moro percutaneous patch test, the Mantoux intracutaneous test, and the Calmette conjunctival test [1]. The first three methods remain in use. By the early 1930s tuberculin skin testing had become a method for screening apparently healthy persons for infection with M. tuberculosis. At that time it was common practice to give a series of four or five graded doses of tuberculin before excluding the possibility of tuberculous infection. As the use of the tuberculin skin test increased, evidence accumulated that not all reactions to purified protein derivative