5,6,7,8-Tetrahydro-8-deazahomofolic acid is a potential thymidylate synthase (TS) inhibitor with inhibitory activity against other folate-related enzymes. 5,6,7,8-Tetrahydro-8-deazahomofolic acid showed mild growth inhibition against enterococci, lactic acid bacteria, and L1210 cells in culture. 5,6,7,8-Tetrahydro-8-deazahomofolic acid also showed weak inhibition against thymidylate synthase , dihydrofolate reductase , glycyl-ribonucleoside aminotransferase, and aminoimidazole-carboxyacyl-ribonucleoside aminotransferase. 5,6,7,8-Tetrahydro-8-deazahomofolic acid showed low substrate activity for thymidylate synthase.