Exogenous fibrinogen has been successfully labeled with 99mTc using a modified electrolytic method. The exact labeling mechanism has not been determined. Experimental data suggest that the labeling process of 99mTc-fibrinogen is quite similar to that of 99mTc-human serum albumin as reported earlier by Benjamin. Technetium-99m-fibrinogen is stable in human plasma or in 1% buffered human serum albumin. A binding efficiency of 76% has been achieved with approximately 25% clottable protein. The entire labeling procedure requires less than 1 hr of preparation time. This short labeling time in a closed system may allow development of a practical method for labeling autologous fibrinogen, thus eliminating the risk of hepatitis transmission.