There is increasing evidence that GH can influence immune function and that it is secreted by lymphocytes. In the present study we investigated the endogenous synthesis and secretion of GH and insulin-like growth factor I (IGF-I) from human thymocytes and evaluated the autocrine/paracrine effects of GH and IGF-I on T cell and thymic epithelial cell proliferation. First, the presence of thymic GH and IGF-I was detected by RIA of thymocyte extracts. Next, using a hormonal enzyme-linked immunoplaque assay, we found that thymocytes secreted GH and IGF-I. Further, we documented the endogenous synthesis of GH by human thymocytes using [35S]methionine labeling followed by immunoprecipitation, gel electrophoresis, and autoradiography. We then evaluated the physiological role of endogenously generated GH and IGF-I. Using an affinity-purified-GH polyclonal antibody, we observed a marked inhibition (P < 0.04) of phytohemagglutinin-stimulated thymocyte proliferation, suggesting an autocrine/paracrine role for the secreted GH. Further, we observed significant (P < 0.001) increases in thymocyte proliferation in cultures stimulated with varying doses of GH and IGF-I. Also, conditioned medium of human thymocytes (1 x 10(5) cells) stimulated with GH for 48 h contained a significant (P < 0.001) amount of IGF-I. Thymocyte proliferation stimulated by GH was significantly (P < 0.01) inhibited by monoclonal as well as polyclonal human IGF-I antisera. Finally, we studied the paracrine effect of thymocyte-secreted GH on human primary thymic epithelial cell (TEC) cultures. A significant (P < 0.05) increase in [3H]thymidine uptake in TEC cultures after GH addition was observed, which was abolished by GH antiserum. Polyclonal and monoclonal IGF-I antisera significantly (P < 0.05) inhibited GH-stimulated TEC proliferation. In summary, human thymocytes synthesize and secrete GH and IGF-I. Further, GH functions as an autocrine/paracrine growth factor in the human thymus via locally synthesized IGF-I.