Membranoproliferative glomerulonephritis (MPGN) may be a component of a generalized vasculitis as well as a component of the clinical expression of type-II mixed cryoglobulinemia (MC). Several studies have established a striking association between hepatitis C virus (HCV) infection and MC. The potential role of HCV in the pathogenesis of MPGN, which occurs in almost half of the cases of MC patients, has not been fully investigated, and the demonstration of HCV proteins as the antigenic constituent of the glomerular immune deposits has remained elusive. Kidney biopsy specimens were obtained from 12 HCV RNA, antibody to HCV (anti-HCV)-positive patients with MPGN and type-II MC, and from 8 controls (3 HCV RNA, anti-HCV-negative patients with MPGN and MC and 5 with noncryoglobulinemic "idiopathic" MPGN). Murine monoclonal antibodies developed against c22-3, E2/NS1, c33c, c100-3, and NS5 proteins were used to detect HCV-related antigens by indirect immunohistochemistry. Acid electroelution of tissue sections was performed to enhance the sensitivity of the immunohistochemical method. Specific HCV-related proteins were detected in glomerular and tubulo-interstitial vascular structures in 8 (66.7%) HCV-positive MC patients and in none of the HCV RNA, anti-HCV-negative controls. HCV immunoreactive deposits displayed the following two major patterns: 1) a linear, homogeneous deposition along glomerular capillary walls, including endothelial cells and sub-endothelial spaces; and 2) a granular bead-like appearance with distinct deposits in mesangial and paramesangial cells. Immunoglobulin G (IgG) and M (IgM) and C3 fraction deposition in adjacent kidney sections displayed features comparable with those found for HCV deposits. Patients with granular deposits showed more pronounced renal impairment and severe proteinuria. These findings indicate that in MC patients with HCV-associated MPGN, kidney deposits consist of HCV-containing immune complexes that are likely to play a direct pathogenetic role in the renal damage.