The screening tests on the myrosinase-like activity of intestinal bacteria of chickens were made employing the cultural analysis of intestinal flora.The tests were made on 68 healthy chickens. A progoitrin prepared from rapeseed was used as substrate, and goitrin in biological fluids was assayed by the spectrophotometry.In the chickens fed with rapeseed oil meai inactivated its myrosinase by cooking, goitrin was detected in the middle intestine, rectum, and blood.The incubation tests indicated that the caecal contents consistently hydrolyzed progoitrin to goitrin, and the middle intestine contents and recrum contents were occasionally active. But, in those of other portions, the myrosinase-like activity could not be found.Moreover, bacteria with myrosinase-like activity were isolated from the caecal contents and caecal feces. These may include various members of the intestinal flora, such as Catenabacterium, Bifidobacterium, Bacteroidaceae, Streptococcus, and Enterobacteriaceae.Thus, these studies.indicated that progoitrin is hydrolyzed by living bacteria which are normal inhabitants of the lower alimentary tract in chickens.