Plasma fibronectin (FN) concentration was measured in patients with idiopathic glomerulonephritis (GN) with or without impaired renal function, in uremic patients undergoing periodic hemodialysis and in renal transplant patients before and after an acute rejection crisis. Results show normal FN levels in idiopathic GN and in renal transplant patients with normal renal function, while significantly lower levels were found in GN with severe renal damage, in uremia before and after dialysis, and in renal transplant patients during acute and chronic graft rejection. Significant correlations between high serum creatinine values and low plasma FN levels were found in renal transplant patients. These findings suggest that the kidney may influence FN levels in the blood since acute (rejection crisis) and chronic renal failure (uremia) cause low concentrations of this protein, while levels tend to return to normal values in patients with uremia after renal transplantation. We hypothesize that the normal kidney removes or perhaps degrades some substances or hormones that may control the release or synthesis of FN. These substances are not dialyzed by cellophane membranes since low plasma FN levels persist after periodic hemodialysis. Only the renal graft provokes an increase of FN in the blood stream.