A fully automated, continuous flow radioimmunoassay system is described. The antibodies are covalently linked to a magnetic cellulose solid phase and separation of bound and free antigen is achieved by applying an external electromagnetic field. The bound fraction, after washing, is counted in a flow-through gamma counter chamber. The system employing Auto Analyzer modules, is highly reproducible and allows assays to be performed without equilibrium being reached, and only 10 minutes incubation time is required. The method is universally applicable to the assay of ligands, either protein, hormone, or drug, and for obtaining information about binding proteins. A new approach to the latter has been developed and is illustrated by a fully automated thyroid hormone uptake test which gives better discrimination between normal and pathological states than existing assays. The system operates at 30 samples/h and is characterised by the high precision, speed, and minimal carry-over which can be achieved.