Seven patients with a form of membranoproliferative glomerulonephritis distinct in its glomerular ultrastructure from other forms are described. The use of silver impregnated electron micrographs revealed contiguous subepithelial and subendothelial deposits associated with basement membrane disruption, replication and layering of lamina densalike material. By light and fluorescence microscopy the appearance was distinctive but not diagnostic. Immunohistology consistently showed abundant C3 and properdin in a granular pattern while immunoglobulins and Clq were variably present. Low serum C3 concentrations were observed at some time in each patient, often accompanied by low levels of properdin, whereas the concentrations of Clq and C4 were normal. The patients were indistinguishable in their clinical course from those with other types of MPGN.