PURPOSE: To isolate a polypeptide chain by treating a ribonucleoprotein complex in the presence of a nonionic surfactant and subsequently treating the dissociated portion of the polypeptide chain in the presence of urea and a cationic surfactant.

CONSTITUTION: A ribonucleoprotein complex is extracted from the tissue extract of an animal and subsequently subjected to an ion exchange chromatography, using an initiation buffer solution containing 0.03-0.3%(w/v) of a nonionic surfactant, to separate a fraction containing a ribonucleoprotein complex derivative where a part of the peptide chain is dissociated. The fraction is mixed with urea and 0.1-1.0%(w/v) of an anionic surfactant, allowed to stand for a constant period to dissociate the target polypeptide chain, followed by subjecting the solution to a molecular sieve chromatography to isolate a polypeptide chain having a mol.wt. of 67000-80000. The polypeptide is employed as an antigen to detect or quantify a ribonucleoproteinresistant antibody existing in a specimen.