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Summary.A histidine loading test was performed on 31 patients before and after saturation with folic acid. These results were compared with pre-treatment serum folate levels. Thirteen patients had normal serum folate levels and urinary histidine metabolite excretion; six patients with low serum folate showed high urinary histidine metabolite excretion which returned to normal after folic acid therapy. Five patients had low serum folate but did not excrete abnormal concentrations of histidine metabolites. Seven patients had normal serum folate but excreted high concentrations of histidine metabolites which were reduced to normal after folic acid therapy.INTRODUCTION.