Abstract: 2-Substituted-1-aziridine-carboxylic acid esters exhibiting immuno-stimulant activity and of the formula ##STR1## wherein X is a carbamoyl or alkoxycarbonyl radical, andR.sup.1 is an aliphatic hydrocarbon radical optionally substituted by halogen, alkoxy, amino, carbamoyloxy, cycloalkyl, hydroxyl, an imido or heterocyclic radical, cycloalkyl; or aryl, aralkyl, aryloxyalkyl or arylthioalkyl wherein the aryl moiety is optionally substituted by halogen, alkyl, alkoxy, hydroxyl, amino, nitro, cyano, acyl, carbalkoxy, thioalkyl, alkylsulphonyl, phenyl or trifluoromethyl. Counterparts where X is --CN and R.sup.1 is as above, except for ethyl, are also new.