1. Extracts containing non-infective soluble antigens have been prepared from the tissues of mice infected with a pantropic or any of three neurotropic strains of Rift Valley fever virus. The antigens were detectable by complement-fixation and gel-precipitin tests using antisera prepared in mice.
2. The extracts appeared to contain at least two antigens separable by electrophoresis and distinguishable by Ouchterlony tests. The faster migrating minor antigen occurs sparsely and was not further examined.
3. No distinction, physical or immunological, was observed between the major antigen derived from the four strains.
4. In gel diffusion-filtration experiments with granulated 7 % agarose, the major antigen appeared to be polydisperse containing particles with diffusion rates ranging from at least as low as that of Burnupena cincta haemocyanin to almost as high as that of haemoglobin.
5. Centrifugal analysis indicated the presence of particles having sedimentation constants of about 8, 29 and greater than 100 Svedberg units. These particles have a density of about 1·27 g./ml. If all the particles of the major antigen are spherical, with a density of 1·27 g./ml., the three size groups have diameters of about 7–8, 14 and < 26 mμ.
6. The diffusion coefficient of the smallest particle was estimated to be 6·06x 10−7 cm.2/sec. corresponding to a sphere 7 mμ in diameter, or 4·78 x 10−7 cm.2/ corresponding to a sphere 9 mμ in diameter depending on which of two values for the diffusion coefficient of mouse γ-globulin is used in the calculation.
7. In density gradient zone electrophoresis at pH 8·6, the principal antigen migrated at about the same rate as haemoglobin and appreciably slower than the virus.
The authors are indebted to Prof. A. Kipps for his continued interest in this work, and to Dr T. H. Mead for valuable criticism of this paper. This investigation was supported in part by a Public Health Service research grant Al 04044–02 from the National Institute of Health, Bethesda, U.S.A.