One daily and 3 thrice weekly retreatment regimens given for 12 months under programme conditions were compared. The daily regimen was rifampicin and ethambutol (RE7). The three intermittent regimens also contained rifampicin and ethambutol: one of them, rifampicin and ethambutol throughout (RE3); the next one supplemented with pyrazinamide for the first 3 months (REZ3); the last one supplemented with prothionamide for the first 3 months (REPt3). The pyrazinamide containing regimen was subdivided into ordinary and high dose groups. The subjects for retreatment were those who have had, at least, more than 6 months of initial triple chemotherapy of isoniazid, PAS and streptomycin at the health centres, and failed to convert to bacteriologically negative status. Among 419 patients who were available for sensitivity tests before commencing retreatment, 393 (94.3%) were resistant to isoniazid. Six hundred and seventy-four patients (674) were allocated randomly to the regimens: 64 patients were excluded due to various pretreatment reasons and 109 did not complete 12 months of chemotherapy. There remain 501 patients who completed their retreatment. As assessed at 12 months, a bacteriologically favourable response was achieved in 68% of 135 RE7 patients, 62% of 129 RE3 patients, 74% of 132 REZ3 patients, and in 79% of 108 REPt3 patients. Adverse reactions were uncommon: 4% in RE7, 5% in RE3 and 9% in REZ3, but 32% in REPt3. Relapse rates during 2 years after termination of chemotherapy were 15% in RE7, 14% in RE3 and REZ3, and 26% in REPt3, as calculated by life table analysis.