The three peptide aldehydes (9a, 9b, and 9c) exhibited antiviral activity. Strikingly, the two potent CatL, B, and S inhibitors 9a and 9b possessed a more pronounced efficacy as antiviral agents with submicromolar EC50 values compared to 9c, which had moderate inhibition. These data pointed to a connection between cathepsin inhibiting and antiviral properties.