A total of 18 birds at the age of 25 days were infected intraocularly with the pathogenic strain 52/70 of the infectious bursitis virus. The bursa of Fabricius and the thymus were sampled for electron-microscopic investigations. The ultrastructural changes in the infected cells consisted in pycnosis of the lymphocyte and macrophage nuclei, vacuolization of the cytoplasm, and the production of lipid droplets as well as of inclusion bodies of varying density, myelin figures, multivericular bodies, and other irregular structures in the cytoplasm. These changes were slightly visible at the 24th hour, were characteristic at the 48th hour, and were well manifested at the 72nd hour following infection. Beside the degenerative changes in the cytoplasm of the affected cells (mainly lymphocytes) there were at the 24th hour of infection tiny-granular matter of fibre-like structure and particles within it of varying electron density. In the later phases (at the 48 h and the 72nd hour) viral particles that were either free or in the vacuoles were predominantly seen in the cytoplasm of the macrophages, having a dia of 50-60 nm and a hexagonal form, with crystalographic arrangement. It was also demonstrated that the replication of the virus of the infectious bursal infection took place in lymphocytes, and in the later stages of infection--chiefly in macrophages. These were the two main types of cells in which virus replication was seen to take place.