Abstract Human caeruloplasmin and α 1 -acid glycoprotein and bovine fetuin were tritiated by reductive methylation of the e-amino group of lysine residues. Tritiated caeruloplasmin retained its oxidase activity and molecular sieve chromatography of the labelled glycoproteins showed that reductive methylation did not significantly affect their molecular size. In experiments with rats, tritiated glycoproteins exhibited plasma half-lives similar to those obtained with the same glycoproteins labelled by other methods. Further, reductive methylation did not alter the property shared by all three glycoproteins to promptly disappear from the circulation after their desialylation and injection into rats and to appear in the liver parenchymal cells. Labelling of glycoproteins by reductive methylation may be useful in metabolic studies requiring the use of both glycoproteins and their asialo-derivatives.