Abstract: A method of manufacturing a bacterial preparation for ruminants which comprises taking out ruminant-stomach bacterial groups from the rumen of a ruminant grown under an excellent condition, repeatedly inoculating by the streak method and culturing the bacterial groups on a same selective medium containing an ammonium salt, gradually increasing the medium while serially passing the bacteria on the medium, allowing certain microorganisms of the bacterial groups to utilize the metabolic products of the bacteria, thereby obtaining bacteria having an improved ammonium-utilizing ability, and then using the improved bacteria as a principal ingredient of a bacterial preparation.