Inhibition of interleukin-1 beta converting enzyme (ICE), apopain, papain, thrombin and trypsin with substrate like peptidyl L- and D-alpha-aldehydes and their L-beta-homo-aldehyde analogues was investigated. The L-beta-homo-aspartals appear to be specific inhibitors for ICE and its homologues; the other enzymes were not inhibited with such L-beta-homo aldehydes. Papain shows tolerance for D-residues at P1 depending on their chiral stability.