Tuberculosis is a prevalent infectious disease that is transmitted primarily from man to man by droplet nuclei. This report describes an outbreak of tuberculosis infection in a hospital in which 8 persons--7 hospital employees and 1 patient--had documented conversion of intermediate strength tuberculin tests from negative to positive in a period of about 10 weeks. The 7 employees constituted 18% of those selected for skin testing because of close contact with a patient who had newly discovered smear-positive pulmonary tuberculosis. The one patient converter had been tested on pulmonary function testing apparatus previously used by the person with active disease; 21 other patients who had also used the contaminated apparatus in the same time period did not convert. Infection of the patient who did convert is attributed to the pulmonary function testing apparatus. Epidemiological investigation revealed the importance of considering factors of inoculum, reservoir, and host in the transmission of infection.