filter assay This assay is performed to study interaction between a radio-labeled ligand and the receptor. The reaction mixture containing the receptor and radioligand is transferred to a filter plate, washed to remove the unbound radioligand. The bound complex remains on the filter, which can be eluted into the scintillation fluid and the radioactivity measured. Filter assays differ from SPA because a separation of free radioligand and radioligand bound to the receptor is required for measurement. scintillation counting