We postulate that 10-formyl-7,8-dihydrofolate (10-HCO-H(2)folate), not 10-formyl-5,6,7,8-tetrahydrofolate (10-HCO-H(4)folate), is the predominant in vivo substrate for mammalian aminoimidazolecarboxamide ribotide (AICAR) transformylase, an enzyme in purine nucleotide biosynthesis de novo, which introduces carbon 2 (C(2)) into the purine ring. 10-HCO-H(2)folate exists in vivo as labeled 10-formyl-folic acid (10-HCO-folic acid: an oxidation product of 10-HCO-H(4)folate and 10-HCO-H(2)folate) and is found after doses of labeled folic acid in humans or laboratory animals. The bioactivity of the unnatural isomer, [6R]-5-formyltetrahydrofolate, in humans is explained by its in vivo conversion to 10-HCO-H(2)folate. The structure and active site of AICAR transformylase are not consistent with other enzymes that utilize 10-HCO-H(4)folate. Because 10-HCO-H(4)folate is rapidly oxidized in vitro to 10-HCO-H(2)folate by cytochrome C alone and in mitochondria, it is hypothesized that this process takes place in vivo. In vitro data indicate that 10-HCO-H(2)folate is kinetically preferred over 10-HCO-H(4)folate by AICAR transformylase and that this enzyme may not have access to sufficient supplies of 10-HCO-H(4)folate. Methotrexate blockage of the AICAR transformylase process in patients with rheumatoid arthritis suggests that dihydrofolate (H(2)folate) reductase is involved and is consistent with H(2)folate and 10-HCO-H(2)folate being the product and substrate for AICAR transformylase. The labeling of purine C(2) by an oral dose of [6RS]-5-H[(13)C]O-H(4)folate in a human subject is consistent with 10-H[(13)C]O-H(2)folate formation from unnatural isomer, [6R]-5-H[(13)C]O-H(4)folate, and it being a substrate for AICAR transformylase. In vitro exchange reactions of purine C(2) using H(4)folate coenzymes are not duplicated in vivo and is consistent with H(2)folate coenzymes being used in vivo by AICAR transformylase.