Abstract: A peptide optionally in isosteric form wherein a methylene group in the backbone chain is disubstituted, one or both substituents being fluorine and/or chlorine, in free form or in pharmaceutically acceptable salt form, such as a compound of formula I ##STR1## wherein the substituents have various significances, or an isosteric form thereof,is useful as an enzyme inhibitor. In particular, as a renin inhibitor, it is useful in the prophylaxis or treatment of hypertension and congestive heart failure.It is prepared by a process comprising the step of coupling two corresponding peptide residues optionally in isosteric form, or precursors thereof, and if required appropriately converting any resultant compound in precursor form.