A study of the nature of the fatty materials in buttermilk and the methods that might be employed to test the product for fat is important from several standpoints. The creamery operator continually asks, "What is the best method of testing buttermilk for fat?" ; the research worker is unable to correlate existing data, since different testing methods are employed by different workers; comparisons of different testing methods have been published without any serious attempt having been made to explain the differences which exist between them; "fat" analyses ranging from 0.10 to 0.65 percent may be obtained with the same sample of buttermilk if different testing methods are employed; the Rose-Gottlieb method, which has been accepted as a standard for the determination of fat in milk and milk products, has been seriously questioned as a testing method for buttermilk by certain workers; and finally, fat loss studies are frequently expressed as buttermilk tests, a fact which makes them useless for purposes of comparison.