Growth hormone plays a significant role in regulation of the humoral and cellular immune responses in physiological as well as pathological situations. This role is exerted via the existence of specific receptors on cells of the immune system. Using flow cytofluorometry and biotinylated bovine GH, we have recently analyzed the expression of GH receptors (GHRs) in murine lymphoid organs. GHRs are widely expressed in all murine hematopoietic tissues and in fetuses, newborns, and 3- and 7-week-old animals. In the bone marrow, all hematopoietic lineages express variables levels of GH receptors, whereas in the thymus, this expression is 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 are GHR positive, with a wider receptor expression on B cells and macrophages (approximately 50%) than on T cells (approximately 20%), where the labeling is seen on both CD4+ and CD8+ cell subsets. Interestingly, the proportion of GHRs bearing CD4+ and CD8+ splenocytes is increased after T-cell activation with Con A or anti-CD3. Finally, all peripheral T cells expressing GHRs also express prolactin receptors. These data provide a molecular basis to study the factors controlling GHR expression and regulation of immune function.