1. The two diastereoisomers of 5-methyltetrahydropteroylglutamate, the form of folate that predominates in food and bile, were prepared chemically. 2. Their intestinal absorption was compared by means of the urinary-excretion method of 3 H-labelled compounds and it was found that the naturally occurring biologically active isomer was excreted to a greater extent. Studies of the clearance of the two forms after intravenous injection showed that this difference was due to preferential intestinal absorption of the naturally occurring isomer. 3. In four patients with coeliac disease, the absorption of the naturally occurring isomer was depressed to the same range as the inactive isomer. 4. Since the two diastereoisomers are chemically inseparable, these results indicate that a carrier-mediated process is involved in the human intestinal absorption of this form of reduced folate.