Summary:Rat cerebral cortex was used to prepare a suspension of benzodiazepine receptors. The suspension was mixed with an extract of specimen and [3H]flunitrazepam. The mixture was filtered through a membrane, and radioactivity on the membrane was measured. Clonazepam concentrations in patient plasma specimens determined by radioreceptor assay and gas‐liquid chromatography agreed well. However, diazepam equivalent concentrations determined by radioreceptor assay were close to the sum of diazepam and desmethyldiazepam concentrations determined by high‐performance liquid chromatography, as desmethyldiazepam had binding activity for the receptor. This receptor assay method is accurate, simple, requires less than 0.5 ml of plasma, and is therefore suitable for analyzing numerous samples in a short time.