An agar medium containing 10% defatted milk has been tested as a diagnostic medium for Pseudomonas aeruginosa and, in particular, for differentiating between P. aeruginosa and P. fluorescens. Only P. aeruginosa colonies gave clear zones due to hydrolysis of casein together with diffused green pigment. Results on milk agar correlated well with the pattern of results from a variety of conventiona...