Elevated sympathetic activity can modulate parameters of immunity. We investigated the role of the low sympathetic activity in resting healthy volunteers by treating them with the beta-adrenergic antagonist propranolol (3 x 40 mg/d for 7 days). Propranolol treatment increased the number of circulating T cells but not that of other white blood cells. Similarly, Con A-stimulated lymphocyte proliferation and IL-2 formation were enhanced. Although the number of circulating NK cells did not change, NK-cell activity was reduced markedly after propranolol treatment. These alterations are not a mirror image of the changes observed under conditions of elevated sympathetic activity but demonstrate that withdrawal of the endogenous sympathetic tone by drugs such as propranolol can modulate parameters of human immunity.