The effect of adding a once-weekly dose of levamisole for 4 or 8 wk to a 6-month antituberculosis chemotherapy regimen has been studied in a double-blind, placebo-controlled clinical trial. All patients received the same daily chemotherapy regimen of streptomycin, isoniazid, rifampin, and pyrazinamide for 8 wk followed by thiacetazone and isoniazid for 18 wk, the first 10 wk being given fully supervised on an inpatient basis in hospital and the remainder to be self-administered at home. In 504 patients with fully susceptible strains pretreatment assessed at the end of chemotherapy, there was no evidence of an advantage to the patients receiving levamisole for 8 wk (L8 series) or 4 wk (L4 series) when compared with those receiving placebo only (L0 series) in terms of the rate of achievement of sputum culture negativity or of radiographic improvement at independent assessment. The bacteriologic relapse rates in 2 yr of follow-up were also similar in the three series, namely, 8% for the 144 patients in the L8, 5% for the 152 in the L4, and 8% for the 160 in the L0 series. There was thus no evidence of benefit from levamisole in the dosage schedule studied. The frequency of adverse reactions to the antituberculosis drugs was unaffected by levamisole. None of 424 patients receiving levamisole had adverse reactions attributable to the drug.