We investigated the mechanisms underlying the increase in mononuclear leukocyte (MNL) beta 2-adrenergic receptor (AR) number and responsiveness after acute infusion of catecholamines. Infusion of isoproterenol and epinephrine, but not of norepinephrine, acutely increased MNL beta-AR density, and this was blocked by the beta 2-selective antagonist ICI 118,551 but not by the beta 1-selective antagonist bisoprolol, suggesting a beta 2-AR-mediated effect. Infusion of isoproterenol but not of norepinephrine also induced a lymphocytosis, with an increase in the number of circulating suppressor/cytolytic T (Ts/c)- and natural killer (NK)-cells but a decrease in helper T (Th)-cells, leading to a decreased Th-Ts/c-cell ratio. beta-AR density was higher in Ts/c-cells than in Th-cells. After isoproterenol infusion, beta-AR density was elevated in all lymphocyte subsets but not in monocytes or platelets, suggesting a lymphocyte-specific phenomenon. Infusion of isoproterenol in splenectomized patients did not alter lymphocyte subset composition and only slightly increased beta 2-AR density. In healthy subjects lymphocyte proliferation in response to various mitogens was attenuated after infusion of isoproterenol but not of norepinephrine; this effect was abolished in splenectomized patients. We conclude that the elevated MNL beta-AR density after acute exposure to beta-adrenergic agonists is caused by a release of lymphocyte subsets from the spleen into the circulation and/or by an exchange of lymphocyte subsets between the spleen and the circulation, whereby freshly released splenic lymphocytes appear to carry more beta-AR than those found in the circulation. This appears to impair immune responsiveness in a dual manner, by decreasing the Th-/Ts/c-cell ratio and by rendering lymphocytes more sensitive to the antiproliferative effects of catecholamines via a higher beta-AR density.