A COMPOUND with the optical spectrum of a ferrihaemochrome was produced when ferricyanide-oxidised phenylhydrazine was added to a solution of ferrihaemoglobin¹. The three isomers of methylphenylhydrazine similarly resulted in ferrihaemochromes, but 4-hydrazinobenzoic acid did not². The induction of haemolytic anaemia by a substituted phenylhydrazine was related to the reactivity of its oxidised form with ferrihaemoglobin to produce a ferrihaemochrome^2,3. Further studies of the reaction of oxidised arylhydrazine with ferrihaemoglobin have established that the formation of a ferrihaemochrome-like product and the character of the optical spectrum of this product depend on the nature and position of substituents on the benzene ring of phenylhydrazine.