It has been known that cyanothiamine obtained by reacting thiamine with BrCN in alkaline medium is reversed to thiamine by cysteine. In the treating cyanothiamine with cysteine, 95% of cyanothiamine was reversed to thiamine at pH 4-6 and at 37℃ for 1 hour. With Taka-diastase cyanothiamine was not reversed to thiamine but reversed with Grampositive spore forming bacillus contaminating in Taka-diastase. However this fact showed that cystine in media was reduced with this strain to cysteine which took part in this reaction.