Peptide-substituted heterocyclic compounds having the formula below, pharmaceutically acceptable salts thereof and intermediates therefor; processes for their preparation and use as immunostimulant and antiinfective agents. ##STR1## wherein R is a 5- or 6-membered N-containing heterocyclyl moiety which can have an additional hetero atom selected from N, S or O; R.sup.1 is hydrogen or (C.sub.1-4)alkyl; x is 0 or an integer from 1 to 5; and y is 0 or 1; provided that when y is 0, said N-containing heterocyclyl moiety is linked to the ##STR2## group at its N atom.