(5-Methyl-2-oxo-1,3-dioxolen-4-yl)methyl 6-[(hexahydro-1H-azepin-1-yl)methyleneamino]penicillanate of the following formula (I) ##STR1## or its pharmaceutically acceptable acid addition salt. The compound is useful as an antibacterial agent and may be prepared by reacting (5-methyl-2-oxo-1,3-dioxolen-4-yl)-methyl 6-aminopenicillanate of the following formula (II) ##STR2## or its acid addition salt with chloro-N,N-hexamethyleneformiminium chloride, and as required, converting the resulting compound to its pharmaceutically acceptable acid addition salt.