1. By means of a new technique, using the “closed bowl” of the Sharpies-Super Centrifuge, in which the distance which the virus particles have to travel under the influence of a given centrifugal force until they are caught mechanically in an agar-gel lining the wall of the bowl amounts to only a few tenths of a millimetre, the potency of 5 c.c. of a foot-and-mouth disease virus suspension can be reduced by 90–99 per cent in 3 min. using a centrifugal force of 24,000 times gravity. The virus can be recovered without loss by extraction of the agar-gel either in fresh medium or even in the liquid layer or “supernatant”, thus showing that adsorption phenomena play no role whatever in the effects obtained by centrifugation.

2. The non-adsorbability of the virus by the agar, however, is determined by the presence of a certain though very small amount of impurities presumably protein which seem to have a “protective” effect. The limit of nonadsorption of the virus by the gel appears to be in that region of “purity” where the sulphosalicylic acid test for protein is negative whereas serological precipitation reactions for host protein are still positive. Virus purified beyond this limit is strongly adsorbed by an agar-gel.

3. Measurements of the sedimentation rate were made on three strains of virus of different type Vallee A, Vallee 0 and Waldmann C and no differences between the three strains were observed. The average diameter of the virus particles determined from the sedimentation rate data was 20 mμ. All the evidence now available for the monodispersity of the virus is discussed. The fact that the infective particles are uniform in size indicates that the measurements recorded here concern actually the single virus elements.

4. The sedimentation equilibrium of the virus has been estimated. The value calculated on this basis for the weight of a single virus particle is circ. 1 × 10−17 g. The diameter of a sphere of this weight and density 1·30 would be 25 mμ. The size values for the virus, 25 mμ estimated from sedimentation equilibrium data and 20 mμ estimated from sedimentation rate data, are discussed in relation to the possible shape of the virus particles.

5. The virus can be sedimented in sucrose and sucrose-salt solutions of densities up to 1·28. On the basis of the sedimentation rates in these media the specific gravity of the virus particles is estimated to be about 1·40. This value however may be abnormally high due to the dehydrating effect of the high concentration of salt or sugar. The application of the centrifuge technique described here to the concentration and purification of the virus of foot-andmouth disease in conjunction with ultrafiltration methods will be the subject of a second paper by Galloway & Schlesinger (1937).