Characterization test of the ability of an enterobacterium to deaminate phenylalanine to phenylpyruvic acid a property peculiar to the Proteus md Providencia species. Performed by adding ferric chloride to a culture in buffered DL-phenylalanine, green colour indicating positivity. The phenylalanine may be included in a yeast extract agar or alternatively in malonate – phenylalanine fluid medium, allowing performance of both the malonate and phenylalanine tests on one culture.