Hemolytic disease characterized by slight anemia, reticulocytosis, hemosiderosis, extramedullary hematopoiesis, and positive indirect antiglobulin (Coombs') tests and renal disease with proteinuria, hypoalbuminemia, and glomerular lesions were produced in Swiss mice by neonatal intraperitoneal inoculation of cell-free filtrates prepared from the spleens of old NZB/Bl mice. Lymphoid cell and plasma cell hyperplasia as well as hypergammaglobulinemia occurred in some of these inoculated mice. Type "C" murine oncogenic virus-like particles, indistinguishable from those previously described (1), were shown by electron microscopic study to be present in distinctive locations, notably in the basal foldings of convoluted tubules in the kidneys of a newborn NZB/Bl mouse, old NZB/Bl mice, a CBA x NZB F(1) hybrid mouse, and a Swiss mouse inoculated with NZB/Bl spleen cell-free filtrate. These observations point to two (perhaps related) circumstances which may be requisites for the pathogenic action of this newly discovered virus within and outside the strain NZB: infection of newborn, or infant, mice; persistent, possibly tolerant, infection of adult mice.