In order to investigate the effect of human growth hormone on T lymphocytes, we utilized a clonogenic assay for mitogen-responsive human peripheral blood T lymphocytes. Lymphocytes were purified by density gradient centrifugation and incubated in the presence of phytohemagglutinin using a two-layer agar technique for CFU- T- lymphocyte culture. Nanogram concentrations of human growth hormone, ovine prolactin, human chorionic somatomammotropin, or growth hormone fragment were added to cell cultures. Growth hormone potentiated normal T-cell colony formation in a species-specific manner. Cells from a homogeneous T-lymphoblast line derived from a patient with a T-cell variant of hairy cell leukemia also showed augmentation of colony growth in the presence of human growth hormone. These studies provide evidence for a direct effect of growth hormone on normal and some neoplastic human T cells.