Evidence has been presented by others that betaMSH immunoreactivity in human plasma is due to beta and gamma lipotropin rather than betaMSH. We have studied this question in normal subjects utilizing a sensitive human betaMSH radioimmunoassay capable of quantifying betaMSH in unextracted plasma with a sensitivity of 7.5 pg/ml. Purified human beta lipotropin cross-reacted 30% on a molar basis with synthetic human betaMSH in this assay. ACTH related peptides showed less than 0.1% cross-reactivity. Normal values at 0800 h were 19.6+/-2.4 pg/ml and suppressed to 9.3+/-1.3 pg/mol following dexamethasone. Metyrapone increased 0800 h values to 379.6+/-89.9 pg/ml. Chromatographic studies on Sephadex G-50 demonstrated no betaMSH per se in human pituitaries, plasma from metyrapone treated normals, patients with Cushing's disease. Nelson's syndrome, or Addison's disease. betaMSH immunoreactivity showed the elution pattern of beta lipotropin.