The immune system and the neuroendocrine system have been shown to be functionally interactive. The neuroendocrine system can modulate the immune response and immune mediators can influence the neuroendocrine system. The present paper focuses on the capacity of lymphocytes to produce and secrete neuroendocrine substances. Lymphocytes can secrete the neuropeptide beta-endorphin in response to activation with mitogen or antigen. Moreover, mediators that are involved in the adaptation to stress have also been shown to induce the release of immunoreactive-beta-endorphin by lymphocytes. It is shown here that stimulation of human peripheral blood mononuclear cells with the beta-adrenergic agonist isoprenaline induces beta-endorphin secretion. The effect of isoprenaline can be mimicked by elevation of the intracellular concentration of cAMP with forskolin or (Bu)2cAMP. Inhibition of cAMP-dependent protein kinase PKA by the antagonist N-[2-(methylamino)ethyl]-5-isoquinoline-sulfonamide abrogates isoprenaline-induced secretion of immunoreactive-beta-endorphin by peripheral blood mononuclear cells. The present data give evidence that, beta-adrenergic activation activation of lymphocytes stimulates the secretion of ir-beta-endorphin via a protein kinase A-dependent mechanism. Both beta-adrenergic agonists as well as beta-endorphin have been shown to modulate the immune response. The data presented here are indicative for a role of beta-endorphin in the modulation of the immune response after beta-adrenergic activation.