 A subsequent dose-ranging study to determine the optimal dose of daily rifapentine during the first 8 weeks of TB treatment was performed by the TBTC. In Study 29X, adults n 331 with sputum-smear-positive pulmonary TB were randomized and received or rifampin 10 mgkgdose or rifapentine 10, 15, or 20 mgkgdose; maximum dose, 1500 mg given with a high fat meal daily 7 days per week for 8 weeks, in addition to isoniazid, pyrazinamide, and ethambutol. At completion of the intensive phase, negative cultures on solid media were noted in 52 of 64 subjects in the rifampin group 81; 62 of 67 subjects in the rifapentine 10 mgkg group 93; not significant; 59 of 66 subjects in the rifapentine 15 mgkg group 89; not significant; and 54 of 57 subjects in the rifapentine 20 mgkg group 95; P .05. Negative cultures in liquid media in the modified intention-to-treat analysis group occurred in 36 of 64 subjects in the rifampin group 56; 50 of 67 subjects in the rifapentine 10 mgkg group 75; P .04; 46 of 66 subjects in the rifapentine 15 mgkg group 70; not significant; and 47 of 57 subjects in the rifapentine 20 mgkg group 83; P .01. High-dose daily rifapentine in combination with standard anti-TB drugs was well tolerated and highly active in surrogate bacteriologic tests. 