Studies have been carried out in mice to examine by gamma camera imaging and dissection analysis the biodistribution of 131I- and 111In-labelled avidin and streptavidin. This has included mice with human tumour xenografts pre-targetted with biotinylated anti-tumour monoclonal antibody. Both iodine-labelled avidin and streptavidin were cleared rapidly from the circulation particularly to kidneys, which showed prolonged retention of the tracer. Similar distribution was seen with 111In-labelled preparations, although here tracer also accumulated in the liver. Radiolabelled preparations could form complexes in vitro with a biotinylated monoclonal antibody. This biotinylated antibody, following radioiodine labelling, localized in xenografts of a human osteosarcoma. When xenografted mice were injected with biotinylated antibody followed by 111In-labelled avidin, levels of tracer per gram of tumour tissue were four times higher than those in non-pretreated mice. However, there was still marked liver and kidney uptake of tracer and, since this dominated the images, tumours were not visualized by scintigraphy.