Studies on the bacteriology of butter cultures have shown that two types of bacteria are normally present, one which attacks primarily the lactose with the formation of large amounts of lactic acid, while the other is characterized by its production of volatile acid from citric acid. These findings suggest the development of butter cultures under carefully controlled conditions by making mixtures of the two types of organisms. While many satisfactory butter cultures have been secured with this procedure, mixtures made with organisms that appear typical on the basis of morphology, cultural characteristics and the usual biochemical tests, often fail to develop a satisfactory aroma and flavor and, accordingly, the percentage of the attempts that are successful is small.