Infrared (IR) absorption measurements have been carried out with gelatin films containing watersoluble silver salts at high concentrations to detect interactions between the silver ion and the peptide group. The principal effect of silver ion on the IR spectrum of gelatin was to shift the amide I band (carbonyl stretching) toward smaller wavenumbers. The effect was maximum when the number of silver ions was about one half that of peptide groups. These results suggest that silver ions in gelatin formed weak C=O...Ag⁺...O=C-type bonds with peptide carbonyl groups. Silver salts added in excess tended to separate from gelatin. A similar effect was observed with a model substance Nmethylacetamide (MAA), which itself formed a crystalline adduct with silver nitrate at a molar ratio of 2: 1.