STUDIES ON THE ALLERGIC AND ANTIGENIC ACTIVITY OF SONIC FILTRATES OF BRUCELLA ABORTUS. I. Live, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pa. Sonic filtrates of Brucella abortus were prepared by exposing dense suspensions of the organisms for two hours to vibrations of audible frequency at 9,000 cycles per second. The magnetostriction oscillator of Chambers and Flosdorf was used. Rabbits and guinea pigs were sensitized by an injection of about 900,000,000live organisms. An intravenous injection of the organisms into rabbits resulted in a more uniform skin sensitivity to the intracutaneous injection of sonic filtrate than an intraperitoneal injection. Guinea pigs were sensitized by an intraperitoneal injection of the organisms. The animals usually showed satisfactory allergic skin sensitivity to the sonic filtrate when tested one month after injection. In some instances an intracutaneous injection of as little as 0.0015 mg of protein in the filtrate produced skin reactions in sensitized animals. The allergic skin reactions were usually at their height 48 to 72 hours after injection of sonic filtrate, and persisted in some cases for as long as 10 to 14 days. The infected animals developed agglutinins, precipitins, and opsonins as determined by serological tests. Single intracutaneous injections of as high as 0.05 mg of protein in the sonic filtrate did not sensitize non-infected rabbits to subsequent skin tests. However, they developed agglutinins as a result of the injection of filtrate, and the agglutinative titers as well as the length of time for which they persisted were directly proportional to the quantity of protein injected. The stability of sonic filtrates of B. abortus was studied by comparing the allergic activity of lyophilized material with that of non-lyophilized filtrate. Tests at frequent intervals over a period of two years did not show any change in the activity of the untreated filtrate. Of three preservatives used, 0.5% phenol, 0.25% formalin, and 1:10,000 dilution of merthiolate, none was found to impair the allergic activity of sonic filtrate in the course of comparisons over a period of one year.