In the present study, we investigated the production of insulin-like growth factor I (IGF-I) by leukocytes and its production after treatment with GH. Immunoreactive (ir) IGF-I was observed in leukocytes by direct immunofluorescence with fluorescein isothiocynate-conjugated antibodies to IGF-I. Studies using immunoaffinity purification, HPLC and a fibroblast proliferation bioassay suggests that the de novo synthesized leukocyte-derived irIGF-I is similar in mol wt, antigenicity, and bioactivity to serum IGF-I. We also evaluated the effect of GH on the production of leukocyte-derived irIGF-I. Spleen cells cultured for 24 h in the presence of exogenous GH caused a 2fold elevation of irIGF-I as demonstrated by RIA and immuno-fluorescence. In order to determine if leukocyte-derived irGH can stimulate the production of irIGF-I, we cultured spleen cells for 24 h in the presence of antibodies specific for GH. The data showed a decrease in the number of cells positive for irIGF-I, suggesting that leukocyte-derived irGH may stimulate the synthesis of irIGF-I by leukocytes. We also demonstrated that exogenous IGF-I can decrease the levels of leukocyte GH-related RNA and ir protein. Taken together, our data demonstrate the synthesis and secretion of bioactive irIGF-I from leukocytes and suggest a regulatory circuit for leukocyte-derived irGH and irIGF-I within the immune system. © 1991 by The Endocrine Society.