Through the hydrolysis of casein, a cultural characteristic has been established in the preliminary identification of Serratia marcescens. Compared against 438 strains of the families, Pseudomonadaceae and Enterobacteriaceae, this organism has a reproducible capability to decompose casein, while most other Gram-negative bacilli failed to produce any hydrolysis, or, at the most, hazy zones only. Routine use has shown this to be a valuable differential aid in the identification of Serratia marcescens.