Synthetic human beta endorphin shows a biphasic effect on the production of specific anti-herpes viral antibodies in vitro. At higher concentrations antibody production is reduced, at lower concentrations it is enhanced. In a proportion of donors whose cells do not produce specific antibody when cultured with antigen alone, the lower concentrations of beta endorphin allow antigen-driven specific antibody production to occur. Specific anti-herpes viral antibodies are also made by natural killer (NK) cell-depleted populations from the same non-producer donors. beta endorphin also exerts biphasic effects on NK, although the direction of the modulation is a mirror image of the effects on antibody production. This suggests that the immunomodulatory actions of beta endorphin on specific immune responses are mediated in part by effects on NK suppressor-inducer cells.