The Amplex® Red/UltraRed stop reagent is designed for use in conjunction with both the Amplex® Red and Amplex® UltraRed fluorogenic substrates and assay kits. The Amplex® Red/UltraRed stop reagent provides convenience and control by allowing the fluorescence signal-generating reaction to be terminated at a user-determined time point. After addition of the stop reagent, the fluorescence signal remains stable for at least 3 hours.