The regulation of GTP cyclohydrolase I, the enzyme that catalyzes the first step in the biosynthesis of the pteridine portion of the folic acid coenzyme has been investigated in Escherichia coli K12AB3292, a p-aminobenzoic acid auxotroph. It was discovered that in extracts made from bacteria grown in the presence of methionine a 3 to 7 fold decrease in specific activity of the enzyme was observed compared to extracts of organisms grown in the absence of this compound. No other folate metabolites, with the exception of serine, exerted similar effects. It was also discovered that a twofold derepression was observed when the organisms were grown on limiting pAB with no folate metabolites as compared to organisms grown in excess pAB under the same conditions. Methionine is capable of overcoming this derepression and causing a 2 to 3 fold repression in limiting pAB extracts as compared to the origanisms grown in excess pAB without methionine.