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THE experiments recorded in this paper deal with the centrifugation of the virus of foot-and-mouth disease in the Sharples-Super Centrifuge by means of a new technique outlined already by one of us (Schlesinger, 1936). The questions discussed are, the application of the method to the estimation of the particle size and particle weight of the virus from the sedimentation rate and the sedimentation equilibrium, the information obtained about the specific gravity of the virus and the use of the method for preparative purposes. The experimental procedure and the results obtained are given in detail, the theoretical explanation of the principles underlying the method and its application to different problems are given briefly. The results of experiments recorded in the present paper were referred to already in general terms in communications made to the International Society for Microbiology at the Congress in London, July 1936. (See Congress Proceedings.) METHODSThe apparatus and method of use. The apparatus employed is the experimental model of the Sharples-Super Centrifuge with a "closed bowl ". This "closed bowl" is a steel cylinder of about 20 cm. length and 4-5 cm. diameter with a removable base and rotating with its axis vertical. The speed used in the present experiments did not exceed 31,000 r.p.m., which corresponds to a centrifugal force of about 24,000 times gravity at the periphery of the cylinder.The cylinder is warmed and 5 or 10 c.c. of liquid agar are introduced and spun for about 20 min. The centrifugal force distributes the agar solution as a thin layer on the wall of the cylinder which as it cools becomes coated with a uniform gel. Now the rotation is stopped and the virus suspension is introduced. In the spinning cylinder the liquid system spreads as a thin film on the surface of the agar layer. The virus particles have, under the influence of the centrifugal force, to travel only the very short distance represented by the thickness of the liquid layer before reaching and entering the agar-gel. When the centrifuge is stopped, the liquid freed to a great extent from the virus flows down and collects at the bottom while the virus particles remain in the agar-1 This work was done on behalf of the Foot-and-Mouth Disease Research Committee who have given their permission for the publication of this paper. One of us (M. S.) wishes to express his thanks to the Academic Assistance Council for a grant.