A comparison has been made of the relapse rates between 7 and 30 months for four 6-month regimens. Streptomycin plus isoniazid plus rifampicin had a relapse rate of 2 per cent of 171 patients; isoniazid plus rifampicin had a relapse rate of 7 per cent of 164. An initial 2 months of streptomycin, isoniazid, rifampicin, and pyrazinamide, when followed by thiacetazone plus isoniazid, had a relapse rate of 7 per cent of 179, and when followed by streptomycin plus isoniazid plus pyrazinamide twice a week the relapse rate was 4 per cent of 159 patients. Of the 36 relapses, 32 occurred in the first 6 months after stopping chemotherapy and 35 were with drug-susceptible organisms. The difference between the relapse rate on the streptomycin plus isoniazid plus rifampicin regimen and the isoniazid plus rifampicin and thiacetazone plus isoniazid regimens approached significance (P =0.06 for both comparisons).