Novel pyroglutamic acid derivatives (I), wherein R.sub.1 is --OH, --ORa, wherein Ra is alkyl, cycloalkyl, alkenyl, cycloalkenyl, aryl, aralkyl or heterocyclyl; R.sub.2, R.sub.3 and R.sub.4 are independently H, a nitrogen protecting group which hydrolyzes under acidic conditions or phtalamide; X is a pharmaceutically acceptable anion; and Y is a N-containing group; either in the form of their isolated optically active stereoisomers or in the form of mixtures thereof, are useful compounds for enhancing an immuneresponse in a subject and/or for treating tumors, bacterial, fungal or viral infections, or autoimmune diseases. ##STR00001##