Abstract The role of enkephalins, beta-endorphin, or other neuropeptides produced by the nervous system in the alteration of immune responsiveness is generally unknown. The present studies were undertaken to investigate the role of these neuropeptides in the modulation of human spontaneous cytotoxicity induced by natural killer (NK) cells. Natural cytotoxicity was measured by using a standard 51Cr release assay with radiolabeled K562 cells. NK activity was significantly enhanced by both beta-endorphin (30.5 +/- 11.5%, M +/- SE, relative enhancement at 50:1, effector:target (E:T) ratio, 10(-14)M beta-endorphin) and methionine-enkephalin (met-enkephalin) (27.4 +/- 9.7% relative enhancement at 10(-9)M). The magnitude of relative enhancement significantly correlated with increasing concentrations of beta-endorphin. Leucine-enkephalin, alpha-endorphin, and morphine did not augment NK activity. The enhancement of NK activity with beta-endorphin increased at all E:T ratios tested. Naloxone inhibited the augmentation of NK activity produced by beta-endorphin and met-enkephalin. By using a combination of a standard 51Cr release and soft agarose single cell analysis assays, beta-endorphin increased both the number of E:T cell conjugates and the number of active killer cells among target-binding cells. The maximal effector cell recycling capacity was increased by 170%. These studies provide new insight into the mechanisms by which neuropeptides produced by the nervous system can alter immune responsiveness.