The structure of aeruginosin B is now defined as 2-amino-6-carboxy-10-methylphenazinium-8-sulphonate by the synthesis of the demethyl derivative. Aeruginosin B could not be synthesised by quaternisation of the demethyl compound or its derivatives, but has been made by the action of sulphite ion on aeruginosin A. Some limitations on the cyclisation of 2-aminodiphenylamines to phenazines in boiling nitrobenzene are described as well as a new phenazine synthesis.