The chemical synthesis of 11-oxahomofolic acid (2) has been carried out using an unambiguous procedure. Reaction of methyl p-hydroxybenzoate with 0-propiolactone gave 3-[p-(carbomethoxy)phenoxy]propionic acid (7), which was converted to l-bromo-4-[p-(carbomethoxy)phenoxy]-2-butanone (8) by the Arndt-Eistert procedure. Protection of the carbonyl group of 8 as the oxime resulted in the formation of 10, which on reaction with potassium phthalimide in the presence of crown-18 ether as a catalyst gave l-phthalimido-4-[p-(carbomethoxy)phenoxy]-2-butanone oxime (11). Hydrazinolysis of 11 gave l-amino-4-[p-(carbomethoxy)phenoxy]-2-butanone oxime (4), which was used as the key intermediate for the construction of 11-oxahomofolic acid (2) by modifcations of the Boon and Leigh procedure. The dithionite reduction product of 2,7,8-dihydro-ll-oxahomofolic acid, served as a substrate of Lactobacillus casei dihydrofolate reductase and exhibited a relative rate of 50% of the natural substrate under identical conditions. The catalytic reduction product of 11-oxahomofolic acid consisting of a mixture of diastereomers exhibited powerful antifolate activity against both MTX-sensitive and -resistant L. casei and Streptococcus faecium. The enzymatic reduction product of 7,8-dihydro-ll-oxahomofolate having the "natural" configuration at C6 exhibited good antifolate activity against both MTX-sensitive and -resistant strains of L. casei and S. faecium. This paper details the synthesis and preliminary biological evaluation of an antifol, which is a substrate of L. casei dihydrofolate reductase in its 7,Sdihydro form and the resulting enzymatic reduction product capable of inhibiting the growth of the same organism from which the enzyme was derived. Thus, 7,8-dihydro-ll-oxahomofolic acid has been shown to be potentially capable of inducing a "lethal synthesis" in L. casei.