Tetrahydrofolate-dependent enzyme activities were investigated upon erythrocyte lysates from two cases with formiminotransferase deficiency syndrome. The activities of both N^{5, 10}-methylene-tetrahydrofolate dehydrogenase and serine-hydroxymethylase were found to be slightly increased. Cyclohydrolase and formate-activating enzyme were found to be normal in their activity.
The results obtained suggested that a metabolic pathway of tetrahydrofolates, including tetrahydrofolate→N^{5, 10}-methylene-tetrahydrofolate→N^{5, 10}-methenyl-tetrahydrofolate→N¹⁰-formyltetrahydrofolate, was of an increased activity in erythrocytes in which formiminotransferase was of defective activity.