Mononuclear inflammatory cells in renal biopsies from 36 patients with membranous nephropathy (MN) were analyzed, using monoclonal antibodies. In the interstitium, monocytes/macrophages and T cells were the predominant cell types (210 +/- 27 and 171 +/- 25/mm2, respectively); in contrast, very few intraglomerular leucocytes, mostly macrophages (1.0 +/- 0.7 cell/glomerular cross-section), were found. Among the interstitial T-cell population, helper/inducer cells (CD4+) predominated (CD4:CD8 ratio, 2.2 +/- 1.5). Natural killer (NK) cells and B lymphocytes were a minor component of the interstitial infiltrates and were almost absent in the glomeruli. Significantly higher numbers of DR-expressing cells were found in the interstitium (322 +/- 20/mm2) than in controls (109 +/- 30), but tubular DR expression was similar to controls (17 +/- 12 mm2). The numbers of total leukocytes and their subsets CD4+, CD8+, monocytes/macrophages, and B cells all correlated with the degree of renal impairment at the time of biopsy, but surprisingly there was no correlation between interstitial cell numbers and the histological severity of tubulointerstitial lesions. Progressive renal impairment over 5 years was associated with many interstitial T cells and monocytes/macrophages in the initial biopsy. Our results suggest that interstitial mononuclear cells may be important determinants in the pathogenesis of MN. Both cellular and humoral immune mechanisms may play a major role in the initiation of the disease, whereas progression toward renal failure seems to be determined mainly by cell-mediated immunity.