Aziridine-2-carboxylic acid derivatives of the formula ##STR1## wherein X is a carboxyl, nitrile, alkoxycarbonyl or carbamoyl group, andR and R.sup.1 are various organic radicals; or pharmacologically acceptable salts thereof, exhibit marked immunostimulant activity, especially in conjunction with added chemotherapeutic agents such as a penicillin, a cephalosporin, a nitrofuran or chloramphenicol. Those compounds are new where X is a cyano group or an alkoxycarbonyl radical and R.sub.1 is a hydrogen atom, but R' is not an unsubstituted alkyl radical or an alkyl radical substituted by hydroxyl, alkoxy, dialkylamino, phenyl, 4-chlorophenyl or 4-methoxyphenyl or a vinyl radical substituted by a phenyl or methyl radical, or a cycloalkyl radical, a phenyl, a 4-chlorophenyl, a 4-methoxyphenyl, an s-triazinyl or a pyridinyl radical; or where X is a carbamoyl group and R.sub.1 is a hydrogen atom, R' is not an unsubstituted cyclohexyl, alkyl or benzyl radical; or where X is a cyano group or an alkoxycarbonyl radical and R.sub.1 is a phenyl radical, R' is not an isopropyl, cyclohexyl, phenyl, benzyl or p-chlorobenzyl radical; or where R.sub.1 is a methyl radical, R.sup.1 is not a benzyl, p-chloro- or p- methoxybenzyl radical.