A medium ("Y" medium) is described, which was more efficient for the isolation of Yersinia enterocolitica from experimentally infected faecal specimens than desoxycholate-citrate. McConkey, lactose-sucrose-urea (LSU) agar, and Yersinia selective medium (Wauter's medium). The "Y" medium consists of casein hydrolysate and peptone servings as carbon and energy sources. A high selectivity is achieved by its contents of sodium oxalate and bile salts. The oxalate suppresses growth of gram-negative rods, including members of the family Enterobacteriaceae and of Pseudomonas spp., while the bile salts inhibit growth of gram-positive bacteria. In the few instances coliform rods grew on the "Y" medium, they could easily be distinguished by their fermentation of lactose, included in the medium, and the fact that colonies of organisms were surrounded by an opaque zone of precipitated bile salts. The most optimal condition for the isolation of Y. enterocolitica from stools was achieved at incubation of the "Y" medium at 29 degrees C for 2 days.