Abstract 6-(N-(substituted-imidoyl)aminoacetamido)penicillanic acids and their salts are valuable as antibacterial agents, nutritional supplements in animal feeds, therapeutic agents in poultry and animals, including man, and are especially useful in the treatment of infectious diseases caused by Gram-positive and certain Gram-negative bacteria, most particularly those caused by the Proteus genus. 6-(N-(Phenylacetimidoyl)aminoacetamido)penicillanic acid, a preferred embodiment of the invention, is prepared by Raney nickel hydrogenation (50 p.s.i., R.T.) of an aqueous solution of sodium 6-(3-benzyl-1,2,4oxadiazole-5-one-4-acetamido)penicillanate which is prepared in turn by reaction of 6-aminopenicillanic acid with 3-benzyl-1,2,4oxadiazole-5-one-4-acetyl chloride.