20S Reaction Buffer contains the optimal formulation for the quantitative analysis of constitutive 20S and immuno 20S proteasome activity. This buffer can be used as supplement to the 20S Proteasome Assay Kit (K-900) or with any individual proteasome enzymes (E-350, E-360, E- 370, E-375) in conjunction with the fluorogenic peptide Suc-Leu-Leu-Val-Tyr-AMC (S-280) and 20S proteasome activation solution (B-30) to measure activity.