6-[.alpha.-(Imidazolidin-2-on-1-ylcarbonylamino)-substutited acetamido]penicillanic acids, and the correspondingly 7-substituted ceph-3-em-4-carboxylic acids, characterized by the presence of a methyleneamino or substituted methyleneamino group on the 3-nitrogen atom of the imidazolidine ring are antibacterial agents. The compounds, of which 6-[.alpha.-(3-benzaliminoimidazolidin-2-on-1-ylcarbonylamino)cyclohexa-1,4-dien-1-ylacetamido]penicillanic acid and 7-[.alpha.-(3-furylideneaminoimidazolidin-2-on-1-ylcarbonylamino)phenylacetamido]-3-(3-methylthiadiazol-5-ylthiomethyl)ceph-3-em-4-carboxylic acid are typical examples, are prepared through acylation of an 6-[.alpha.-(amino)substituted acetamido]penicillanic acid or the corresponding 7-[.alpha.-(amino)substituted acetamido]ceph-3-em-4-carboxylic acid with a reactive nucleofugic derivative of a 3-methyleneaminoimidazolidin-2-on-1-carboxylic acid.