NCC-ST-433 monoclonal antibody raised against human gastric carcinoma xenograft (St-4) was labeled with 125I using enzymatic and Iodogen methods. While labeling efficiency of the antibody was more excellent by enzymatic method, specific radioactivity of the antibody labeled by Iodogen method was higher than that by enzymatic method. The labeled antibody was stable in vitro and in vivo, and the labeled NCC-ST-433 was specifically accumulated in NCC-ST-433 antigen positive human tumor cell lines in vitro. The specificity of 125I-NCC-ST-433 in vivo was found to be more excellent when this antibody was labeled by Iodogen method and actually excellent images of H-69, a human small cell lung carcinoma, were obtained 5 days after injection of 7 micrograms of 125I-NCC-ST-433 per mouse. This method seemed to be promising for imaging human lung small cell carcinomas.