The intrinsic thyrotropic activity of hCG purified from normal pregnancy urine has been demonstrated in several laboratories. hCG has a specific thyrotropic potency of about 0.04-0.5 microU bovine (b) TSH/IU hCG, depending on the bioassay system. The corresponding potency for hCG derived from pathological sources, such as hydatidiform moles and choriocarcinoma tissue, or from the serum of these patients has not been studied as extensively. Since the biological activity of glycoproteins can be strongly influenced by variations in the oligosaccharide side-chain composition, we have investigated the effect of anion exchange chromatography on the thyrotropic potency of hCG derived from the hydatidiform mole and serum of three hyperthyroid patients with molar pregnancy. The activity for the fraction of total molar hCG immunoreactivity that was not retained by an anion exchange column (0.18-0.90 microM bTSH/IU hCG) was about twice that of the corresponding serum and molar hCG fraction eluting during the NaCl gradient elution (0.08-0.40 microU bTSH/IU hCG). The unretained hCG fraction corresponds to a previously described hCG precursor that is partially desialated in the C-terminal region of the beta-subunit.