Five 6-month antituberculosis regimens, allocated at random to patients with acid-fast bacilli in their sputum on microscopy, were studied. Four, given 3 times a week throughout, contained isoniazid and rifampicin together with 1. streptomycin, pyrazinamide and ethambutol, 2. streptomycin and pyrazinamide, but no ethambutol, 3. streptomycin and ethambutol, but no pyrazinamide, 4. pyrazinamide and ethambutol, but no streptomycin. The fifth was a daily regimen of isoniazid, rifampicin, pyrazinamide and ethambutol. All 833 patients with drug-sensitive strains of tubercle bacilli pretreatment had a favourable bacteriological response during chemotherapy, and the bacteriological relapse rate during 18 months after stopping chemotherapy was 1% for the three-times-weekly regimens containing streptomycin and pyrazinamide in addition to isoniazid and rifampicin (regimens 1 and 2, above) and for the daily regimen, 2% for the regimen of isoniazid, rifampicin, pyrazinamide and ethambutol three times a week (regimen 4), but 8% for the only regimen which did not contain pyrazinamide (regimen 3). The results achieved by the 4 pyrazinamide regimens were practically as good for the 110 patients with bacilli resistant to isoniazid, streptomycin, or both drugs pretreatment as they were for the patients with drug-sensitive strains.