SUMMARY A method is described for the radioimmunoassay of β-melanocyte-stimulating hormone (β-MSH) in human plasma. It was capable of detecting 20–30 pg β-MSH and was unaffected by the presence of α-MSH and human adrenocorticotrophic hormone. However, cross-reactivity did occur with β-glutamyl MSH (porcine). A simple technique employing porous glass (Florisil) was used to extract β-MSH from plasma. In normal male subjects plasma β-MSH levels ranged from 21 to 133 pg/ml. In patients receiving cortisol therapy for Addison's disease slightly elevated levels were found. Much higher levels were found in patients who had undergone bilateral adrenalectomy as treatment for Cushing's disease.