The ability of Bacillus and some Mycobacteria and Nocardia to digest casein is a secondary characterization test performed by growing the strains on a mixture of equal parts of sterilized skimmed or dialysed milk and double strength nutrient agar. Incubation is up to 2 weeks; hydrolysis is shown by clearing of the opaque medium around colonies, which should persist despite flooding the plate with acid mercuric chloride.