The serum half-life of somatomedin (SM) activity has been measured following the intravenous injection of SM activity into hypophysectomized rats. A [³H]thymidine incorporation assay in chick embryo fibroblasts (CEFs) has been utilized to measure SM activity. Cell cycle analysis data obtained with the flow microfluorometer shows that the [³H]thymidine incorporation data reflects actual DNA synthesis. When normal rat serum was injected, a half-life for SM activity of approximately 3 h was determined. In marked contrast, when serum from hypophysectomized (hypox) rats acutely treated with growth hormone (GH) was used as the source of SM actively, the half-life of the SM actively was short, approximately 8 min. However, when serum from hypox rats chronically treated with GH was injected, the half-life was again long, approximately 4 h. SM activity has been separated from its binding proteins by boiling under acid conditions or chromatography on Sephadex G-50 in 1 n acetic acid. The halflife of this partially purified SM activity was short, in the range of 10–30 min. Finally, recombination of the partially purified SM activity with the large proteins in normal serum extended the half-life of the SM from 10–30 min to about 2 h. These data indicate that the serum half-life of SM activity is GH dependent and suggest that the serum binding protein, like SM itself, is under GH control.