We have analyzed the expression of GH receptors (GHR) in murine lymphoid organs using flow cytofluorometry with biotinylated bovine GH and specific fluorescein isothiocyanate-labeled monoclonal antibodies defining distinct lymphoid cell populations. GHR were widely expressed in al murine hematopoietic tissues and in fetuses, newborns, and 3- and 7-week-old animals. In the bone marrow, all hematopoietic lineages expressed variable levels of GH receptors, whereas in the thymus, this expression was mainly seen in CD4-, CD8-, CD4+CD8+, and CD8+ subpopulations. At the periphery, 50% of splenocytes and peripheral blood lymphocytes and 20% of lymph node cells were GHR positive, with a wider receptor expression on B cells and macrophages (approximately 50%) than on T cells (approximately 20%), where the labeling was seen on both CD4+ and CD8+ cell subsets. Interestingly, the proportion of GHR-bearing CD4+ and CD8+ splenocytes significantly increased after T cell activation with Concanavalin A or anti-CD3. Finally, we demonstrated that all peripheral T cells expressing GHR also expressed PRL receptors. Our study provides a molecular basis to study the factors controlling GHR expression and to better understand the influence of GH in the regulation of immune function.