Peptide inhibitors, specifically labeled at the α-amino terminus by dansylation, have been prepared by utilizing solid-phase peptide synthesis. Changes in fluorescence have been observed upon mixing these peptides with porcine pepsin that can be attributed to the formation of at least two complexes. Energy transfer between tryptophan residues of the protein and the dansyl group of the inhibitors has been detected by the unique excitation spectra generated. The kinetics of forjnation of the second complex can be correlated with inhibition of the catalytic activity of pepsin. Evidence for complex formation has also been obtained from gel filtration experiments using the fluorescent peptides.