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Aim. The purpose of the work was to determine the effect of trichloroacetic (TCA) and perchloric (HClO4) acids on the result of ninhydrin reaction with various amino acids. Methods. A standard method of amino acid detection using a ninhydrin reagent was applied. Optical spectra and density of reaction products were determined spectrophotometrically. Results and conclusions. As a result, it was found that the studied acids change the spectral characteristics of the products of the ninhydrin reaction with amino acids. TCA significantly reduced the optical density of chromophores, and HClO4 also led to a significant shift of the spectra of the reaction products into the short-wavelength region. An exception was the reaction with proline, as a result of which a well-defined maximum appeared in the product spectrum: ?= 620 nm in the presence of TCA and ?=515 nm with HClO4. At the same time, in the presence of HClO4, the reaction became highly specific for proline. Conditions. The ninhydrin reaction with proline upon addition of HClO4 were analyzed in detail. As a result, a new method of highly specific determination of proline in the presence of other amino acids was proposed.