Two 6-month daily regimens of chemotherapy for smear-positive pulmonary tuberculosis were compared in Tanzania. Both had the same initial 2-month intensive phase of streptomycin, isoniazid, rifampicin, and pyrazinamide, and the continuation phase was either thiacetazone plus isoniazid or isoniazid alone. All patients were hospital inpatients for 6 months, solely to ensure that chemotherapy was fully supervised throughout. The patients were followed up to 24 months after stopping chemotherapy. In patients with fully sensitive strains pretreatment, there were no failures during chemotherapy on either regimen; the bacteriologic relapse rates were 3% for the 105 patients receiving thiacetazone plus isoniazid in the continuation phase, and 11% for the 100 patients receiving isoniazid alone (p less than 0.05). Possible adverse reactions were reported in the initial phase in 5 (1.6%) of 319 patients who started treatment, and in 2 of 306 who started the continuation phase, chemotherapy being modified in 5 of the 7 patients.