Thiamine propyl disulfide (I) and its homologuss having benzyl (II), hydroxyethyl (III), tetrahydrofurfuryl (IV) and 8-(methyl 6-acetyldihydrothioctate)(V), as alkylmercapto radicals, were found to be more potent inhibitors to urease than thiamine-disulfide (VI), when these compounds were preincubated with urease at the pH range higher than 8. The inhibition was highest with (II) and in decending orders of (V), (III), (IV) and (I), but the inhibition by (I) was much higher than that by (VI). These inhibitions were shown to be progressive with duration of preincubation and easily recovered or prevented by addition of an excess of cysteine. S-allylmercapto- or S-propylmercapto-L-cysteine was proved to be an inhibitor, weaker than (I) but stronger than (VI). Other disulfide compounds such as cystine, lipoic acid and oxidized forms of glutathione and thioglycollic acid showed no inhibition to urease after the preincubation with these compounds. Asymmetric thiamine disulfide compounds can react with active mercapto groups of the enzyme molecule, resulting in a presumed inactive S-alkylmer-capto-urease compound.