Mice of the inbred NZB strain develop a spontaneous disease characterized by autoimmune hemolytic anemia, positive lupus erythematosus cell tests and antinuclear antibodies and nephritis. This disease is analogous to human systemic lupus erythematosus. In ultrastructural studies of the glomerular lesion in NZB mice, intraglomerular dense deposits in mesangial, subepithelial and subendothelial locations were described. In common with the findings in many examples of human and experimental nephritis, including many cases of human lupus nephritis, these deposits were amorphous or slightly granular in appearance with no definable substructure.We have recently observed structured deposits in the glomeruli of NZB mice. They were uncommon and were found in older animals with severe glomerular lesions by morphologic criteria. They were seen most commonly as extracellular elements in subendothelial and mesangial regions. The deposits ranged up to 3 microns in greatest dimension and were often adjacent to deposits of lipid-like round particles of 30 to 250 millimicrons in diameter and with amorphous dense deposits.