Intercellular adhesion molecule-1 (ICAM-1) plays an important role in immune responses. Expression of this molecule was examined on cryostat sections of 15 normal human and 112 glomerulonephritic kidneys using a specific monoclonal antibody and the indirect immunoperoxidase staining technique. The expression of ICAM-1 on renal structures was compared to that of HLA-DQ, -DR, -DR/DP and -DP antigens. On normal kidneys ICAM-1 was observed on some Bowman's capsular cells and on single glomerular cells which probably represent endothelial and mesangial cells. ICAM-1 was present on peritubular capillaries and on vascular endothelium of large vessels as well as on fibroblasts, whereas no expression of ICAM-1 on proximal tubular epithelial cells (PTECs) was detected. In normal renal tissues the distribution of ICAM-1 was similar to that of HLA-DQ and -DP antigens. In kidneys with different forms of glomerulonephritis especially in association with interstitial inflammation, an abnormal expression of ICAM-1 on PTECs was frequently correlated with aberrant expression of HLA-DQ and -DP antigens. These results further support the hypothesis that PTECs may participate in cell-mediated immune reactions in glomerulonephritis.