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A peptide with insulin-like activity was isolated from human plasma. In the purification, insulin-like activity (ILA) was monitored by radioreceptor assay for insulin, using human placental membrane and [125I]-insulin. ILA was extracted from Cohn fraction III with acid-ethanol and chromatographed on Sephadex G-75 in 1% formic acid. When the active fractions were subjected to ion-exchange chromatography with CM-cellulose, the ILA was adsorbed to the column at pH 5.0 and was eluted with a gradient of ammonium acetate. The chromatographic behavior of the ILA was not identical to that of somatomedin A as determined by radioreceptor assay (RRA) for the latter. On isoelectric focusing of the ILA from the CM-cellulose column, insulin-like activity was distributed over a wide pH range. The ILA that was focused at pH 7.5-9.0 was further purified by gel filtration on Sephadex G-50 in 1 M acetic acid. The specific activity of the basic ILA was approximately 200 mU insulin equivalent /mg protein. The apparent molecular weight of the material was estimated to be 7,000. It stimulated [14C]-glucose oxidation in rat epididymal fat cells and had sulfation activity in chick chondrocytes. Furthermore, the basic ILA had a potent mitogenic activity in Balb/c-3T3 cells. Thus, the basic ILA is qualified as an 'insulin-like growth factor' (IGF). It is obviously different from somatomedin A, and may be closely related to IGF-I or somatomedin C. Subsequently, RRA for the basic ILA was developed in serum concentration of the basic ILA were determined. The serum concentrations of the basic ILA were high in acromegalics and low in patients with hypopituitarism. Thus, the basic ILA is entitled to be one of the IGFs.