The main objective of this research is to aid in the design of new antitumor and antibacterial agents based on: 1) the metabolism of diastereoisomers of tetrahydrohomofolate, 5-methyltetrahydrohomofolate, 5-methyltetrahydrofolate, 2) the properties of the enzymes thymidylate synthease, dihydrofolate reductase and serine hydroxymethyltransferase, their kinetics and interaction with ligands including pteroylpolyglutamates, 3) the evaluation of folate and dihydroneopterin analogues as enzyme inhibitors and inhibitors of bacterial growth. The following methods are to be employed: 13C nuclear magnetic response, enzyme kinetics, microbiological assay, radioactivity assays and affinity chromoatography.