There is already a considerable amount of evidence suggesting that fibronectin (Fn) plays an important role in the pathogenic process in some forms of glomerulonephritis (GN). It has been postulated that Fn may participate in the progression or regression of glomerular diseases. The Fn is presented in the kidney as a normal component of the mesangium, and it is increased in the expanded mesangium in various forms of GN. This paper reports our efforts to investigate the role of Fn in plasma and kidney in patients with GN. Using monoclonal antibodies against human Fn in the ELISA and immunohistoperoxidase techniques to evaluate Fn, we investigated its quantity in connection with clinical state and morphological findings. We studied 93 patients with GN and 26 renal biopsies. The patients with active forms of mesangial proliferative, membranoproliferative and membranous GN showed increased plasma Fn, and the highest levels were in patients with nephrotic syndrome. Increased tissue Fn correlated with mesangial expansion and with IgG and C3 deposits. We speculate on possible mechanisms of the involvement of Fn in human chronic GN.