ICR-nude mice inoculated intraperitoneally with MHV-NuU, showed progressive proteinuria and had virus in the kidney at 2 to 5 weeks postinoculation. In the renal glomeruli, mesangial and endothelial cells were enlarged and proliferated with infiltration of neutrophils, mononuclear cells and a few multinucleated giant cells. Proliferative endovasculitis was seen also in the brain, lung, liver, pancreas, omentum and mesentery. Immunofluorescence revealed MHV antigen within endothelial cells of the small veins, and multinucleated giant cells and glomerular mesangial cells. Segmental or diffuse deposition of immunogloblin as well as C3 were present in glomerular mesangial areas in association with MHV-antigen. By electron microscopy were seen degeneration of podocytes and hypertrophy of endothelial cells with subendothelial or paramesangial deposits. Virions were seen within vascular endothelial cells, macrophages and giant cells in and around the small vein but not within those in the glomeruli.