Concanavalin-A covalently linked to Sepharose was used to extract the glycoproteins from a volume of human plasma about 10 times that volume usually added to conventional radioimmunoassay systems. Using a chorionic gonadoptropin (hCG) radioimmunoassay, hCG inhibition curves generated in the presence of the extracts were nearly identical to those generated by the conventional method; therefore, the sensitivity of the radioimmunoassays for glycoprotein hormones.