The value of selected cultures of bacteria for the development of a desirable flavor and aroma in butter has been thoroughly established through comparisons of butter made with and without culture and also through the extensive commercial use of cultures. There are two distinct types of organisms present in the butter cultures commonly used, one of which (Streptococcus lactis or, according to certain investigators, Streptococcus cremoris) attacks primarily the lactose with the formation of large amounts of lactic acid while the other type (Streptococcus citrovorus and Streptococcus paracitrovorus Hammer (4), or Betacoccus Cremoris Knudsen and Sorensen (11), or Leuconostoc dextranicus and Leuconostoc citrovorus Hucker and Pederson (9)) is characterized by the fermentation of citric acid. The growth of the two types in a butter culture results in the formation of various compounds that are responsible for the more or less characteristic flavor and aroma. Volatile acids constitute one of the groups of these compounds that has been studied extensively, particularly in connection with the differentiation of the butter culture organisms (4, 6). The volatile acids do not account for the desirable aroma of butter in which culture is used, although they may be a factor in this connection. Recent studies carried out in Europe indicate that diacetyl, a compound that is readily derived from acetylmethylcarbinol by oxidation, is of special significance from the standpoint of the desirable aroma of butter.