A semisolid medium (designated Serratia differentiation medium) containing L-arabinose, ornithine, and selective inhibitor was used to differentiate three clinically encountered Serratia species. The inhibitor, Irgasan DP-300, was incorporated to eliminate false-positive reactions from most remaining Enterobacteriaceae. The suspected Serratia colony was inoculated as a stab into the medium. Serratia marcescens was indicated by a change in color from olive to purple following 18 h of incubation, whereas S. rubidaea (not listed in Bergey's Manual of Determinative Bacteriology) was indicated by a change to bright yellow. S. liquefaciens (described in Bergey's Manual of Determinative Bacteriology [8th ed., 1974] as Enterobacter liquefaciens) produced a small purple band at the top of the medium and a yellow or yellow-green butt. Absence of growth and color change following incubation indicates that the suspected colony is a non-Serratia. Thirty-six Serratia strains and 97 other Enterobacteriaceae and Pseudomonadaceae strains were tested. Two strains of the non-Serratia Enterobacteriaceae (one each of Citrobacter freundii and Proteus morganii) and two strains of Pseudomonas aeruginosa produced a color change in the medium. All of the Serratia strains tested were correctly identified using this medium, while 96% of the other species tested were inhibited.