Distribution of interleukin 6 (IL-6) in normal and diseased kidneys was examined by an immunohistochemical method. Four specimens of normal kidney tissue and 47 specimens obtained from diseased kidneys by biopsy were studied by indirect immunofluorescence microscopy using specific monoclonal antibody to human IL-6. In the normal kidney, IL-6 was distributed in the glomerular mesangial area and in vascular walls. In the diseased kidneys, IL-6 was observed in the glomerulus in various patterns. The extent of the distribution of IL-6 correlated with the average number of glomerular cells. When sclerosis appeared, the expression of IL-6 decreased. Occasionally, IL-6 was found in the area of synechiae or crescents as well as in the parietal epithelial cells of the glomerulus with synechiae and/or crescents. In the interstitium, IL-6 was seen in atrophic tubules, and the distribution correlated with the extent of tubular atrophy. The binding of anti-IL-6 monoclonal antibody to the renal sections was completely inhibited by preincubation of the antibody with recombinant IL-6. These results suggests that IL-6 expression is a good marker for glomerular cell proliferation and that IL-6 may be involved in the formation of synechiae or crescents and in tubulointerstitial injury.