The disclosure describes (S)-3-[2-(2-amino-4-thiazolyl)]-(Z)-2-methoxyiminoacetylamino-2-oxo-1-azet idinyliminoacetic acid and (S)-3-[2-(2-amino-4-thiazolyl)]-(Z)-2-methoxyiminoacetylamino-2-(S)-methyl -4-oxo-1-azetidinyliminoacetic acid and the cationic salts thereof which possess antibacterial activity.