We compared the bacteriologic relapse between 5 and 28 months for five 4-month chemotherapeutic regimens for pulmonary tuberculosis. The regimens were: (1) streptomycin plus isoniazid plus rifampin plus pyrazinamide daily for 8 wk followed by isoniazid plus rifampin plus pyrazinamide daily for 9 wk; (2) the same 4 initial drugs for 8 wk followed by isoniazid plus rifampin daily for 9 wk; (3) the same 4 initial drugs for 8 wk followed by isoniazid plus pyrazinamide daily for 9 wk; (4) the same 4 drugs for 8 wk followed by isoniazid daily for 9 wk; (5) the same as regimen 4; but without streptomycin for the first 8 wk. The first 2 regimens, in which rifampin was given for 4 months, had relapse rates of 16 and 11%, respectively, but the rates were much higher for the regimens in which rifampin was given for only 2 months (32 and 30 %, respectively). The addition of pyrazinamide in the continuation phase had no effect on relapse rate. Removal of the streptomycin (regimen 5) resulted in a relapse rate of 40%, but this was not significantly higher than that (30%) after regimen 4 (p = 0.2).