The mechanisms by which insulin and catecholamines affect low-density lipoprotein (LDL)-receptor activity were studied in freshly isolated human mononuclear leukocytes. Incubation of cells for up to 24 h in a lipid-free medium resulted in an increase in the specific binding, accumulation, and degradation of 125I-labeled LDL. Insulin stimulated the ability of the cells to bind, accumulate, and degrade the lipoprotein with high affinity, which may be caused by an increase in the LDL-receptor number without altering binding affinity. (-)-Epinephrine inhibited the specific binding, accumulation, and degradation of 125I-LDL. This effect appears to be mediated by a decrease in the number of LDL receptors and not by a change in the binding affinity. (-)-Norepinephrine, the unspecific beta-adrenergic agonist (-)-isoproterenol, and the beta 2-specific agonist terbutaline mimicked the effect of epinephrine on LDL-receptor activity. Catecholamines and beta-adrenergic agonists yielded sigmoidal log-concentration effect curves. The action of epinephrine was attenuated by the beta-antagonist (dl)-propranolol. These results demonstrate that insulin stimulates and catecholamines suppress the specific binding, accumulation, and degradation of 125I-LDL in human mononuclear leukocytes. The catecholamine action appears to be mediated by beta 2-adrenergic receptors. A suppression of LDL-receptor activity resulting from deficiency of insulin and elevated plasma catecholamine concentrations in uncontrolled insulin-dependent diabetic patients may contribute to the increased levels of LDL cholesterol observed in these patients.