Oncomodulin (oncoM)──a mark of tumor was purified from human tumor tissues
and was labelled with high specific radioactivity by conjugating to an 125I-iodinisated acylating agent.The specific activity of 125I-oncoM was 0.20-0.34MBq/μg and its radiochemical purity was 96%. Radioimmunoassay showed that the 125I-oncoM was able to combine with oncoM antibody specifically. The binding rate was 40% at antiserum dilution of 1:4000.Competitive inhibition curve was obtained in the presence of unlabelled oncoM.