Tripeptide-derived molecules incorporating N-methyl amino acid residues and C-terminal Michael acceptor moieties were evaluated as irreversible inhibitors of the cysteine-containing human rhinovirus 3C protease (3CP). Such compounds displayed good 3CP inhibition activity (k(obs)/[I] up to 610,000 M(-1)s(-1)) and potent in vitro antiviral properties (EC50 approaching 0.03 mu M) when tested against HRV serotype-14. (C) 1999 Elsevier Science Ltd. All rights reserved.