A radioimmunoassay has been developed for Somatomedin B, a growth hormone-dependent factor that stimulates DNA synthesis in human glia-like cells. The sensitivity permits detection of this factor in human plasma diluted 1: 20,000 and in monkey plasma diluted 1: 5,000. It is not measurable in nonprimate plasma diluted 1: 20. The concentration in growth hormone-deficient adult patients is equivalent to 6.6plus or minus0.5 ug/ml of a highly purified somatomedin preparation. In acromegaly the concentration is 19.3plus or minus2.3 ug/ml and falls after definitive therapy that results in a decrease in plasma growth hormone. In unextracted human plasma the immunoreactive Somatomedin B is associated with a plasma protein at least as large as gamma-globulin and with an electrophoretic mobility on paper resembling the alpha-globulins. The level of Somatomedin B in the bound form in human plasma under steady-state conditions may depend on the rate of production of the peptide and/or the concentration of the plasma-binding protein. At present there is no information concerning which of these is modulated by growth hormone. Immunoreactive Somatomedin B is found predominantly in Cohn plasma fractions III and IV, largely dissociated from the plasma-binding protein. The disappearance curves of labeled purified Somatomedin B and of immunoreactive Somatomedin B from acromegalic plasma administered intravenously to a dog were superposable; the terminal portion of the disappearance curve having a half time of almost an hour.