Disclosed herein are compounds of formula 1 ##STR1## wherein X is a terminal group, for example, an aryloxycarbonyl or an alkanoyl; B is absent or an amino acid residue, for example, Val or Asn; R.sup.1 is alkyl; and Y is a ring substituent, for example, benzyl, benzyloxy, phenylthio or 2-pyridinylthio. The compounds inhibit the activity of human immunodeficiency virus (HIV) protease and interfere with HIV induced cytopathogenic effects in human cells. These properties render the compounds useful for combating HIV infections.