A procedure is developed for the flow-injection spectrophotometric determination of cysteine in dietary supplements based on the formation of a reduced phosphomolybdic complex by the analyte in mixing a sample solution with a solution of ammonium 18-molybdophosphate. For the flow-injection determination of cysteine, a polymethyl methacrylate chip is fabricated, in the channels of which the analytical form was obtained and detected. The detection limit for cysteine is 3 × 10^–6 M and the throughput is 240 determinations per hour. The procedure was tested in the analysis of different samples of dietary supplements; the results obtained were verified by HPLC.