An account is given of immunohistological studies performed on tissue specimens from 59 cattle affected with BVD-MD. The animals had either died or been killed in an advanced state of the disease. From 47 of these animals VD-virus was demonstrated in primary calf kidney cultures inoculated with suspensions of at least one of the following tissues: small intestine, lung, kidney, and spleen. Conjugates were produced from sera of goats that had been immunized with the Danish VD-virus strain UG59. VD-virus could be demonstrated in all the types of tissue examined, but the degree of involvement was particularly high in the mucous membranes of the alimentary tract. It was characteristic that in all tissues viral antigen could often be demonstrated in the walls of blood vessels. In 9 out of 10 cases infected neurons were demonstrated in the cerebral cortex and hippocampus. If fresh tissues are available for examination, FA-staining of cryostat sections may give a rapid diagnosis of BVD-MD.