[en] A method for labeling proteins with ¹¹C has been developed. The method consists of adding ¹¹C-formaldehyde (derived from ¹¹CO₂ by catalytic oxidation of ¹¹C-methanol) to a buffered protein solution, followed by reduction of the resulting formaldehyde-protein adduct with aliquots of sodium borohydride. Separation of the desired product is achieved simply and efficiently using high-speed liquid chromatography or, in the case of fibrinogen, by ammonium sulfate precipitation. This mild technique leads to high yields of labeled protein and is adaptable to the labeling of proteins and peptides or other molecules which contain a free amino group and which are stable at pH 8. (U.S.)