While the tuberculin test has long been a most useful tool for identifying the tuberculous infected in a population, it has the limitation that it may elicit cross-reactions in persons infected with other (nontuberculous) mycobacteria. One approach toward improving the identification of persons with tuberculous infection, is a dual-test technique, using an atypical mycobacterial antigen together with tuberculin. In 1958, the Public Health Service and the US Navy instituted a cooperative program designed to study skin-test sensitivity and related tuberculosis morbidity among navy recruits. Preliminary results from this program provide striking evidence that the use of another mycobacterial purified protein derivative (PPD) in addition to tuberculin significantly improves the identification of the the tuberculous infected in a population.