Monoclonal antibody 1G3.10 (mouse IgG3) against the human urinary bladder carcinoma TSGH-8301 was isolated from hybridoma ascites and labelled with radioiodine. The antibody reacts with a cell surface antigen preferentially expressed in human bladder carcinoma. Binding studies in vitro demonstrated that its specificity for antigen was retained after iodination. The clearance of the radiolabelled monoclonal antibody was not modified by the presence of the tumour. In vivo localisation of the radiolabelled antibody and control normal IgG to tumour xenograft was determined by counting the tissue radioactivity and by external gamma ray scintigraphy with computer analysis of the region of interest at various times after the intravenous injection of radiolabelled antibodies. Maximum tumour-to-blood radioactivity (4.5) was obtained 4 days after antibody injection. There was no tumour localisation of radiolabelled normal IgG. Specificity of the localisation was also confirmed by using a non-reactive colon carcinoma xenograft. Distinct tumour images were obtained without the use of subtraction techniques. These studies of human bladder tumour localisation using this monoclonal antibody show its obvious potential for clinical use.