Abstract: In this work, the conditions of spectrophotometric evaluation of the ninhydrin assay for L-lysine in a water dimethyl sulfoxide solution were investigated. The absorption spectrum of the L-lysine–ninhydrin reaction product exhibited two maxima at 420 and 500 nm. It was shown that pH of the reaction medium affected the intensity of the product color. The stability of the ninhydrin reaction products was the highest at pH 8–9. The phase proportion of the water–organic solution had little effect on the optical density of the product. The absorption spectra of the ninhydrin reaction products of L-lysine derivatives: N^α-boc-lysine and N^ε-z-lysine, indicated that it was the α amino group of lysine that participated in the reaction.