Abstract: Automated radioassay in which a serum is diluted and pre-incubated in the presence of a radioactive labeled ligand, such as an antigen, and a binder, such as an antibody, specific to the ligand. The pre-incubated mixture is flowed through a chamber containing binder specific to the ligand supported on a solid support, and the labeled and unlabeled ligand not bound to the binder in pre-incubation are bound to the receptor on flow through the chamber. An eluting solution is flowed through the chamber to release the ligand bound to the binder in the chamber for reuse thereof. By counting the radioactivity of one or both of the fraction which flows through the chamber or which is subsequently released therefrom the quantity of a specific ligand in the serum may be assayed.