This study was performed to examine the in vivo effects of a bolus of recombinant human growth hormone (r-hGH) on the human immune system. In a double blind placebo controlled cross over study, healthy volunteers were given 2 IU r-hGH as an intravenous infusion. r-hGH did not influence the subpopulations of blood mononuclear cells (BMNC), natural killer cell activity, in vitro proliferative responses or production of interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-2, IL-6, IL-8, tumor necrosis factor alpha (TNF alpha), TNF beta or interferon gamma in supernatants from BMNC stimulated with either lipopolysaccharide or phytohemagglutinin. However, two h after infusion a significant neutrocytosis occurred. It is concluded that a bolus infusion of r-hGH to healthy volunteers exerts only minor effects on the human immune system.