Significance. The sulfur amino acids, Met and Cys, are highly variable in human foods and there are risks associated with both deficiency and excess. Both are required for physiologic processes in addition to maintenance of protein synthesis and nitrogen balance, and, in principle, excess of either could lead to generation of toxic sulfur metabolites in the colon. The present study will test 2 novel approaches to define dietary sulfur amino acid needs (i.e., plasma redox and metabolic analyses by NMR and MS). The results are expected to establish the feasibility of using these approaches for improved definition of Dietary Reference Intake values for these important nutrients.