The biological significance of growth hormone (GH) in the physiology and pathophysiology of the immune system is not established. To address the site and mode of action through which GH exerts its effects on lymphocyte tumors, we applied a well-characterized monoclonal antibody directed against the hormone binding site of the receptor and were able to further characterize the tumor by immunohistochemical localization of GH receptors. Cutaneous T cell lymphomas were identified by histologic and immunomorphologic diagnosis according to the updated Kiel classification, with the application of monoclonal antibodies. Nodular tumors of the skin, identified as highly malignant Ki-1 lymphomas of large anaplastic cells, had intense GH receptor immunoreactivity. The presence of GH receptors in these proliferating tumor cells supports the hypothesis that GH is involved in paracrine-autocrine mechanisms acting locally in regulating peripheral T cell lymphoma tumor growth.