Using a modified fibrinogen-uptake (FUT) test, 22 patients were investigated at various intervals after kidney transplantation. Eight of the patients had developed or showed early clinical signs of acute rejection crisis at the time of measurement. Another fourteen patients formed the control group with no evidence of florid rejection process. Comparison of the 125I-fibrinogen accumulation in the renal grafts between the two mentioned groups showed clear differences with high statistical significance (p less than 0.001). Uptake of labelled fibrinogen was increased in every acute rejection transplant. Histologically the kidneys with increased accumulation of fibrinogen showed extensive deposits of fibrin in blood vessels, glomeruli, intracapillary thrombi and in the interstitium. Except for the limitations discussed in this paper we consider this test to be of a great clinical value in the diagnosis of rejection episodes.