Ten sera from 4 patients with aeromegaly, serum from a normal individual, and purified pituitary growth hormone (hGH) were examined by gel chromatography on Sephadex G-100. At least three components of immunoreactive hGH that differ in their apparent molecular size were demonstrated in all samples. These studies suggest that there may be as many as four circulating components of immunoreactive hGH, two of which elute from Sephadex G-100 before albumin. The data also suggest that at least one of these components, peak III (Kav ∼ 0.4), is secreted both in response to thyrotropin-releasing hormone (TRH) in patients with aeromegaly and in response to insulin-induced hypoglycemia in normal individuals.