A fluorescent human renin inhibitor, dansyl-Phe-His-LVA-Ile-Amp (3, U-80,825), was synthesized and utilized in a fluorescence energy transfer displacement assay to determine the dissociation constants (kd's) of a series of ditekiren analogues. These studies have indicated that (1) both the parent ditekiren (2) and compounds 8a-3 are up to 1 order of magnitude more potent than revealed by their IC50's, these dissociation constants are in good agreement with the independently determined Ki's for compounds 2,3, and 8d, and (3) the lower limit of the fluorescence energy transfer displacement assay has been extended beyond the picomolar range. It has therefore been suggested that many examples of underestimation of renin inhibitory activity may exist in the renin literature which could be discovered and rectified by using the methodology described herein.