A positive tuberculin reaction, which indicates tuberculous infection, is known to correlate with increased rates of tuberculous disease, but current data concerning possible relationships with factors such as ethnic group, socioeconomic status, age, and sex are unpublished or unavailable. Current background rates of tuberculous infection are also generally not available, so that expected prevalence rates of tuberculous infection in urban areas have not been confidently established. In 1973/74, the New York City Health Code presented an opportunity to study these factors in more than 50,000 employees of the Board of Education. For the first time, a specific index was used to quantitate socioeconomic status. The relative effect of each of these variables on reactor rate was measured. It was found that tuberculous infection was related to race or ethnic group, socioeconomic status, age, and sex, in that order. The over-all data presented a recent picture of the prevalence of tuberculous infection in an urban population.