A study of rifabutin in the retreatment of patients with chronic pulmonary tuberculosis whose strains of tubercle bacilli were resistant to all three of the drugs isoniazid, streptomycin, and rifampicin, and usually to others as well, was undertaken in 22 Chinese patients in Hong Kong. They were arranged in 11 pairs such that the resistance pattern of the strains for both patients was the same or closely similar. One of each pair was allocated at random to receive rifabutin (B) daily and the other rifampicin (R) daily. The two members of each pair were also prescribed the same or similar companion drugs, selected on the basis of the results of susceptibility tests and of the history of previous antituberculosis chemotherapy. The bacteriological results showed no evidence of a sustained benefit in any patient. A temporary response was seen on sputum smear examination in 14 patients (7B, 7R), of whom 10 (5B, 5R) had a period of smear-negativity, and on sputum culture examination in 10 patients (5B, 5R), of whom 3 (2B, 1R) had a period of culture-negativity. The duration of response was longer for the B patient in 5 pairs and for the R patient in 5. Two patients (both B) had rifabutin-susceptible strains on admission to the study; their temporary responses were among the best and were associated with the emergence of rifabutin resistance, suggesting that rifabutin may have contributed to their response. A total of 17 patients, including 7 not in the paired comparison, were subsequently retreated with ofloxacin; 10 showed a response, disease becoming and remaining quiescent in 3.(ABSTRACT TRUNCATED AT 250 WORDS)