Severe acute respiratory syndrome coronavirus (SARS-CoV) 3C-like protease (3CLpro) mediates extensive proteolytic processing of replicase polyproteins, and is considered a promising target for anti-SARS drug development. Here we present a rapid and high-throughput screening method to study the substrate specificity of SARS-CoV 3CLpro. Six target amino acid positions flanking the SARS-CoV 3CLpro cleavage site were investigated. Each batch of mixed peptide substrates with defined amino acid substitutions at the target amino acid position was synthesized via the "cartridge replacement" approach and was subjected to enzymatic cleavage by recombinant SARS-CoV 3CLpro. Susceptibility of each peptide substrate to SARS-CoV 3CLpro cleavage was monitored simultaneously by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The hydrophobic pocket in the P2 position at the protease cleavage site is crucial to SARS-CoV 3CLpro-specific binding, which is limited to substitution by hydrophobic residue. The binding interface of SARS-CoV 3CLpro that is facing the P1โ€ฒ position is suggested to be occupied by acidic amino acids, thus the P1โ€ฒ position is intolerant to acidic residue substitution, owing to electrostatic repulsion. Steric hindrance caused by some bulky or ฮฒ-branching amino acids in P3 and P2โ€ฒ positions may also hinder the binding of SARS-CoV 3CLpro. This study generates a comprehensive overview of SARS-CoV 3CLpro substrate specificity, which serves as the design basis of synthetic peptide-based SARS-CoV 3CLpro inhibitors. Our experimental approach is believed to be widely applicable for investigating the substrate specificity of other proteases in a rapid and high-throughput manner that is compatible for future automated analysis. Copyright ยฉ 2006 The Protein Society.