A novel class of tetrapeptide .alpha.-ketoamides useful for selectively inhibiting serine proteases, selectively inhibiting cysteine proteases, generally inhibiting all serine proteases, and generally inhibiting all cysteine proteases, having the formula M.sub.1 --AA.sup.1 --AA.sup.2 --AA.sup.3 --AA.sup.4 --CO--NR.sub.3 R.sub.4.