We tested the hypothesis that some cereal-product vehicles may reduce fortificant bioavailability below 85% and examined the feasibility of using single dose, dual-label, non-saturation protocols for studying bioavailability based on urinary excretion ratios (UER; % oral 13C6 isotope dose excreted in intact folate/% i.v. 2H4 isotope dose excreted). Fifteen females received 225 micrograms oral folate (capsules, fortified bran flakes, and fortified white bread), mainly as 13C6-PteGlu, followed by i.v. injection of 100 micrograms 2H4-PteGlu. UERs were used as the primary index of absorption. Urinary folate was cleaved to p-aminobenzoylglutamic acid, dervivatized and determined by gas chromatography/mass spectroscopy (GC/MS). The UER mean (95%CI) for folic acid was 2.18 (1.2-3.8) at 48 hours and as these were greater than 1.0, it was concluded that oral and i.v. isotopes of folic acid are handled differently by the body. Compared to the 48 hour UER for folic acid, UERs for white bread and bran flakes were 0.71 and 0.37, respectively, thus indicating some matrix inhibition of absorption. Consideration should be given to the choice of cereal-based fortification vehicles in order to maximize bioavailability. Plasma enrichment of folate can be measured using LC/MSMS (liquid chromatography/mass spectroscopy-mass spectroscopy) but seems unfeasible with the GC/MS method.