A pathogenic strain of infectious bursal disease virus has been purified by density gradient centrifugation, principally on sucrose and tartrate gradients. Examination of gradient peak fractions by electron microscopy has revealed two populations of particles of average size 62 nm and 20 nm, which band together on sucrose and tartrate gradients. Purified virus has been shown to reproduce the typical symptoms and lftsions of infectious bursal disease. The possible classification of the larger virus particle in the diplornavirus group and the origin of the smaller particle are discussed.