Abstract: A radioimmunoassay method for the determination of brain antigens or protein such as S-100 responsible for neurological disorders is disclosed. The method involves preparing a standard blood plasma or serum formulation containing substantially none of the protein being assayed and approximately the average degree of non-specific competitive binding found in the unknown blood sample and mixing known quantities of the protein being assayed with the standard blood formulation to provide a reference standard. The unknown blood sample is then mixed with a radioactively labeled sample of the protein being assayed and an antibody capable of immunoreactivity with the protein, the mixture is incubated, the antibody bound protein is separated from the unbound protein and the relative amounts of antibody bound and unbound protein are determined.