Plasmapheresis with a dextran sulfate column is a treatment for patients with hypercholesteremia. When proteins bound to the column during the treatment were fractionated to prepare some known proteins, we found a 57 kDa glycoprotein designated GP57 which showed a new N-terminal amino acid sequence. Western-blot analysis of human plasma revealed that only a 120 kDa protein, GP120, reacted with anti-GP57 antibody. Since GP120 and GP57 had an identical N-terminal amino acid sequence, GP120 is probably the intact form of GP57. The isoelectric point of GP120 was 6.8. N-Glycanase treatment decreased the molecular weight of GP120 by 15 kDa. Neuraminidase and O-glycanase, however, did not affect the molecular weight. Amino acid sequence analyses of the lysylendopeptidase digest of GP120 revealed significant homology to the heavy chains of inter-alpha-trypsin inhibitor (ITI) family. Since GP120 showed no bikunin sequence, and chondroitinase treatment and alkaline treatment of GP120 did not affect its molecular weight, we concluded that GP120 was not a complex with bikunin. We designated GP120 as IHRP (ITI heavy chain-related protein).