The in vivo distribution of the intercellular adhesion molecule ICAM-1 was investigated in renal tissue specimens obtained from 17 renal allotransplanted patients, nine normal kidneys (controls), seven native kidneys, nine patients with mesangioproliferative glomerulonephritis, and nine patients with active extracapillary glomerulonephritis. Biopsies from patients with signs of acute rejection showed a significant increase in ICAM-1 expression in the tubular epithelium (P less than 0.05). In normal kidneys (controls) ICAM-1 expression was found in endothelial cells; additional expression in the tubular epithelial cells was induced in patients with extracapillary glomerulonephritis. The in vitro expression of ICAM-1 was examined in cultured human tubular cells after stimulation with gamma-interferon and interleukin-1, treatment with cyclosporin and/or verapamil and coculture with allogenic mononuclear cells. An increased ICAM-1 expression was demonstrated by coculture with allogenic mononuclear cells and after stimulation with gamma-interferon and/or interleukin-1. Cyclosporin or verapamil induced no changes. Our results give support to the hypothesis that ICAM-1 upregulation is important in immune interactions such as allograft rejection. Furthermore, the in vitro model indicates that ICAM-1 expression is regulated by gamma-interferon and interleukin-1 produced by activated T lymphocytes and macrophages.