The effect on the kidney capillaries of T lymphocyte culture supernatants from patients with minimal change nephrotic syndrome was studied by infusion into the left renal artery of rats. The supernatants of T lymphocyte cultures from nephrotic patients induced significant proteinuria and a reduction of anionic sites in the glomerular basement membrane of the kidneys in the rats. These changes were not produced in rats receiving equivalent supernatants from healthy controls.