The fact that milk may contain organisms of the Brucella group has been known for many years, and this fact has been demonstrated by various investigators. These investigators, in most instances, have resorted to animal inoculation to detect the presence of these organisms, and in many cases have used samples of milk obtained under aseptic conditions from individual cows. The present work was undertaken to answer the following questions : 1. Is it possible to detect organisms of the Brucella group in market certified milk by plating methods ? 2. How many organisms are present per cubic centimeter of milk? A few of the instances in which B. abortus has been reported in milk are reviewed. Schroeder and Cotton,1 in 1911, tested seventyseven samples of milk from thirty-one dairies and found, by inoculation in guinea-pigs, that eight of the samples derived from six different dairies contained B. abortus. Evans,2 in 1915, examined, by plating methods, milk which had been drawn aseptically. Of two certified dairies studied she found fourteen of forty-six samples to contain B. abortus in numbers ranging from 110 to 4,300 per cubic centimeter of milk. In the same year, Fleischner and Meyer3 reported that "B. abortus is, for all practical purposes, always present in the certified milk produced in the San Francisco Bay region." Guinea-pigs were used to demonstrate the organisms in their experiments. Carpenter and Baker,4 in 1927, examined the milk from fifty herds supplying Ithaca, N. Y., and found, by inoculation into guinea-pigs, B. abortus present in the milk from nine of the herds. Other investigators have reported similar results.