MHC class I and II molecules serve as restriction elements for the presentation of antigen-specific T cell reactions and may be implicated in the process of cellular transformation. We have analysed the expression of MHC class I and II antigens including the associated invariant gamma-chain in 100 consecutive renal biopsies by a sensitive immunoperoxidase method using monoclonal antibodies against HLA-ABC, HLA-DR, HLA-DQ, HLA-DP and the invariant gamma-chain (Ii). HLA-ABC was expressed by almost all parenchymal cells except for podocytes. Staining for HLA-DR was found consistently in glomerular and interstitial capillary endothelial cells, in "dendritic" (LCA positive) and most infiltrating interstitial cells but not in mesangial and most extracapillary cells. Similar but weaker positivity was observed for HLA-DQ and -DP. HLA-DR positive infiltrating mononuclear cells in glomeruli were noted in acute glomerulonephritis (GN) and Lupus GN associated with enhanced staining of endothelial cells. In addition, HLA-DR was uniformly or focally expressed by predominantly proximal tubules in 80% of biopsies, especially IgA-GN, focal glomerulosclerosis, acute GN but also in 54% of cases with minimal change nephrosis. HLA-DP and -DQ were only positive in a limited number of HLA-DR positive tubules. The expression of the invariant gamma-chain was comparable with the reactivity for HLA-DR. In addition, all HLA-DR-positive tubules expressed the invariant gamma-chain. Subtyping of T cells in biopsies with DR-positive tubules and interstitial infiltrates (n = 20) showed a predominance of CD 4 positive cells in the majority of cases.(ABSTRACT TRUNCATED AT 250 WORDS)