Proline derivatives of the formulae: ##STR1## wherein R.sup.1 through R.sup.11 have defined values, and acid- and base-addition salts thereof, and equilibrium addition compounds of the aldehyde group thereof; processes for their preparation; pharmaceutical compositions; and intermediates for preparing said proline derivatives. The proline derivatives are human leukocyte elastase inhibitors which are useful, for example, in treating pulmonary emphysema.