During 1995-96, staff from the California departments of corrections and health services as well as local health departments investigated two outbreaks of drug-susceptible TB. In each outbreak, all cases were linked by IS6110-based DNA fingerprinting of Mycobacterium tuberculosis isolates. In both of the investigations, a positive tuberculin skin test (TST) was defined as an induration of greater than or equal to 5 mm in contacts and/or HIV-infected persons. A TST conversion in a contact was defined as an increase of greater than or equal to 5 mm from a documented negative to a positive TST within the previous 2 years. Only culture-positive pulmonary cases were considered infectious, and the infectious period was considered to begin 6 weeks before the date the culture-positive specimen was obtained (if the patient was asymptomatic) or the date of onset of symptoms consistent with TB. The findings show that M. tuberculosis can spread rapidly among HIV-infected inmates and be transmitted to their visitors and prison employees, with secondary spread to the community. A prompt response to infectious TB cases is critical to minimize the transmission of TB and the development of disease among infected persons. All persons suspected to have infectious TB, including any person with a respiratory specimen that is smear-positive for acid-fast bacillus, must be placed immediately in respiratory isolation; a contact investigation must be initiated promptly. All HIV- infected contacts, regardless of TST status, should receive preventive therapy once TB disease is excluded. 7 references