Factors inhibitory to beta-glucuronidase were found in the culture filtrate and in a bacillary extract of Mycobacterium tuberculosis H37Rv grown for 6 weeks on Sauton medium. The inhibitors were purified by ammonium sulfate fractionation, treatment with n-butanol and streptomycin, and chromatography on DEAE-Sepharose CL-6B. Two inhibitors were obtained from the culture filtrate. The molecular weights were estimated to be 25,500 and 15,500 by gel filtration on a Sephadex G-75 column. Three inhibitors were purified from the bacillary extract, two of which were similar to those from the culture filtrate. The molecular weight of the third inhibitor was 21,000. However, the molecular weight of all the denatured inhibitors was 8,600 in the presence of sodium dodecyl sulfate. The inhibitors contained extremely high amounts of glutamic and aspartic acids and had a highly acidic isoelectric point of pH 2.5. The inhibitors acted noncompetitively against beta-glucuronidase of guinea pig origin at an optimal pH 4.5. beta-Glucuronidases from human peripheral leukocytes and beef liver were partially sensitive to the inhibitors; all the other enzymes tested for sensitivity were unaffected by the inhibitors.