A significant impediment to studies of growth disorders in man has been the lack of an easily accessible target tissue for studies of GH action. As human fibroblasts have been shown to produce a somatomedin-C-like peptide in culture, the aim of this study was to investigate the interaction of GH with cultured fibroblasts. GH binding to human skin fibroblasts and to lung fibroblasts was examined. Specific binding of [125I]iodo human GH [( 125I]iodo hGH) was rapid, reversible, and time and temperature dependent. Maximal binding was achieved within 2 h at 30 C and specifically bound [125I]iodo-hGH was rapidly dissociable at this temperature. A linear relationship between specific binding of [125I]iodo-hGH and cell number was found and negligible degradation of labeled hormone occurred during the binding studies. Half-maximal inhibition of specific binding occurred at 30 ng/ml hGH. Lactogenic and nonprimate somatogenic hormones did not displace [125I]iodo-hGH at the concentrations tested. Scatchard analysis of [125I]iodo-hGH binding to human skin fibroblasts demonstrated a single class of binding sites with an affinity constant of 1.07 +/- 0.07 (SEM, n = 5) X 10(9) M-1 and a capacity of 8355 +/- 1880 site per cell. Similar GH-receptor characteristics were found on each of the fibroblast cell lines examined irrespective of the site of origin (skin or lung) or age of donor. These findings demonstrate for the first time specific GH receptors in cultured human fibroblast cell lines. The demonstration of GH receptors in human fibroblasts should encourage the search for similar receptors in tissues not previously considered to be target tissues for GH action. On the basis of these studies we suggest that cultured skin fibroblasts may be a suitable tissue for the study of GH-receptor status in patients with disorders of growth.