The highly reactive ε-NH2 group of the essential amin acid lysine interacts with food constituents, especiailly carbohydrates, to become chemically and nutritionally unavailable. Several assays have been developed to measure reactive (available) and unreactive (unavailable) lysine in proteins. These depend on reaction of chemically accessible ε-NH2 groups with (1) fluorodini trobenzene (FDNB); (2) trinitrobenzenesulfonic acid (TNBS); (3) O-methylisourea (OMI); (4) conjugated vinyl compounds such as methyl acrylate (MA), methyl vinyl sulfone (MVS), ethyl vinyl sulfone (EVS), and phenyl vinyl sulfone (PVS); (5) S-ethyl trifluorothioacetate; (6) acid dyes; and (7) Ninhydrin. An ideal reagent for this purpose should selectively modify ε-NH2 groups under mild conditions to a derivative that is stable to protein acid hydrolysis and that can be assayed by standard amino acid analysis and spectroscopic techniques. Proposed and potential new methods for reactive lysine are critically discussed in terms of these requirements.