Radiolabeling of alpha-fetoprotein monoclonal antibody (AFP-MoAb) with 131I for detection of liver tumor was performed. Liver tumor in Japanese quails was induced by aflatoxin B1, which is potent carcinogen to animals and birds. The labeling of AFP-MoAb was carried out using chloramine-T as oxidizing agent. The effect of the oxidizing agent amount, substrate amount, reaction time, and pH of the reaction medium on the product yield was studied. The labeled compound was quantitatively separated and purified by HPLC. The biodistribution of 131I-AFP-MoAb was studied upon oral administration of the tracer in both normal and tumor-bearing Japanese quails. Localization of 131I-AFP-MoAb in liver tumor indicates that this labeled compound can be used for radioimmunodetection of liver tumor.