PURPOSE: To obtain a polypeptide exhibiting an altered binding affinity for a metal chelate binding antibody useful for diagosing and treating diseases.

CONSTITUTION: This polypeptide comprises an amino acid sequence including a complementary determining region(CDR) sequence of a native antibody and has a binding affinity for a metal chelate of ethylenediaminetetraacetic acid(EDTA) or an EDTA analog or 2-p-nitrobenzyl-1,4,7,10-tetraazacyclododecane-N, N'-N"-N"'-tetraacetic acid(DOTA) or a DOTA analog. The affibity thereof is about equal to or altered from that of the binding affinity of the native antibody.