1. 1. An enzyme sysetm obtained from baker's yeast converted 2-methyl-4 amino-5-aminomethylpyrimidine to 2-methyl-4-amino-5-hydroxymethylpyrimidine, a known intermediate in thiamine biosynthesis. The pH optimum of the system was 7.8 and the apparent Km value for the substrate was 5·10-5 M. The enzyme system was inhibited by 2-mercaptoethanol, pyridoxal phosphate, the amine oxidase inhibitors pargyline and tranylcypromine, and by various carbonyl reagents and chelating agents. 2. 2. Enzymes systems catalyzing a similar reaction were reaction were found to be present in Neurospora crasa extracts and in a crude preparation of Bacillus thiaminolyticus thiaminase.