Compounds of formula I ##STR00001## in which R.sub.1, R.sub.2, R.sub.3, each X, L, Y, Cy, Lp, D and n have the meanings as set out in the specification, and corresponding compounds in which the unsubstituted or substituted amidine group is replaced with an unsubstituted or substituted aminomethyl group, are useful as serine protease inhibitors.