Abstract: Novel .beta.-lactams containing a free carboxylic group of the formula: ##EQU1## wherein A and B are each selected from the group consisting of alkyl, including but not limited to branched alkyl, aryl, aralkyl and monocyclic heterocyclic groups at least one of A and B having a free carboxylic group. Further potential substituents of A and B are members of the group consisting of hydroxy, alkoxy, halogen, nitro, --NH.sub.2, monoalkylamino and dialkylamino groups, X is selected from the group consisting of hydrogen, alkyl, aryl and monocyclic heterocyclic, Y is selected from the group consisting of hydrogen, alkyl and aryl, and Z is selected from the group consisting of alkyl, alkoxy, aryloxy, aralkoxy, alkylthio, arylthio, aralkylthio, --CN, --COO-alkyl and --N.sub.3, useful as antibiotics and for the preparation of polymers and .beta.-amino acids, novel process for their preparation and novel imines used as starting materials for the said .beta.-lactams.