A two-site immunoradiometric assay for a brain specific protein (alpha-albumin or glial fibrillary acidic protein) is described which may be applied to other proteins. The assay variables studied are immunoadsorbent dilution, temperature, incubation times, reactivity of the labelled antibodies and effect of the washing. Each of these factors is optimalised, resulting in a sensitive, specific, reliable assay. The working range is 5--5000 ng/ml, the within-assay variance is 5%. No high dose hook effect is seen in this range. The method is applied for the measurement of alpha-albumin present in cerebrospinal fluid, nervous tissue extracts and cell cultures.