10-Formyl-7,8-dihydrofolic acid (10-HCO-H2folate) was prepared by controlled air oxidation of 10-formyl-5,6,7,8-tetrahydrofolic acid (10-HCO-H4folate). The UV spectra of the 10-HCO-H2folate preparation has λ(max.) 234, 333 nm and λ(min.) 301 nm at pH 7.4, and λ(max.) 257, 328 nm and λ(min.) 229, 307 nm at pH 1. 1H-NMR spectroscopy of 10-HCO-H2folate (in 2H2O; 300 MHz) suggested a pure compound and gave resonances for one formyl group proton, two protons on C-7 and C-9, and no evidence for a C-6 proton, which is consistent with the structure proposed. The spectral properties indicated that the 10-HCO-H2folate preparation is not appreciably contaminated with 10-HCO-H4folate, 5,10-methenyltetrahydrofolic acid (5,10-CH = H4folate) or 10-formylfolic acid (10-HCO-folate). The above data establish that the 10-HCO-H,folate prepared here is authentic. In contrast, a folate with a UV spectrum having λ(max.) 272 nm and λ(min.) 256 nm at pH 7, which was prepared by 2,6-dichloro-indophenol oxidation of 10-HCO-H4folate and reported to be 97% pure, is apparently not 10-HCO-H2folate. 10-HCO-H2folate is utilized by Jurkat-cell (human T-cell leukaemia) and chicken liver aminoimidazolecarboxamide ribonucleotide transformylase (AICAR T'ase; EC 184.108.40.206) in the presence of excess 5-aminoimidazole-4-carboxamide ribotide (AICAR) resulting in the appearance of approximately 1 mol of H2folate product for each mol of AICAR formylated. The present 10-HCO-H2folate preparation had a kinetic advantage over 10-HCO-H2folate resulting from a difference of approx. 5-fold in K(m) values when both folates were used as cofactors for Jurkat-cell and rat bone marrow AICAR T'ase. No substantial kinetic advantage was observed using chicken liver AICAR T'ase. 10-HCO-H2folate had little or no activity with Jurkat-cell or chicken liver glycinamide ribonucleotide transformylase (GAR T'ase, EC 220.127.116.11). The existence in vivo of 10-HCO-H2folate is suggested in mammals by several reports of detectable amounts of radiolabelled 10-HCO-folate in bile and urine after administration of radiolabelled folic acid.