Pathogen-free CD1 mice were experimentally infected with mouse hepatitis virus (MHV3), and half of them were also given daily doses of cortisone. Ultrastructural examination of the splenic white pulp revealed viral particles, corresponding in size, shape and structure to MHV3, within small lymphocytes as well as extracellularly. Virus formation within the lymphocytes was associated with proliferation of the rough endoplasmic reticulum and budding of viral particles from these membranes. Intracellular viral particles were readily seen only in the cortisone-treated mice, possibly because of the stabilising effect of cortisone on intracellular membranes. It is suggested that the lymphoid necrosis of MHV3 infection is attributable to the virus replication within the lymphocytes.