The immune and neuroendocrine systems appear able to communicate with each other by virtue of signal molecules (hormones) and receptors common to both systems. This review summarizes our data concerning the production of one type of neuroendocrine hormone, endorphins (END), by stimulated lymphocytes. The induction by various agents and structural characterization are described, as well as the lymphocyte-derived END's potential role as a mediator of endotoxic shock. Also discussed is the ability of pituitary-derived END and the related enkephalins to inhibit a primary in vitro antibody response. These activities are discussed in the context of a complete regulatory circuit between the immune and neuroendocrine systems.