Infection of lymphocytes with Newcastle disease virus induces the cells to synthesize immunoreactive (ir) adrenocorticotropin (ACTH) and endorphin. The irACTH is synthesized de novo, and common properties of lymphocyte and pituitary ACTH include: antigenicity, bioactivity, molecular weight, and retention time on reverse phase high-pressure liquid chromatography. The irACTH appears to be active in vivo because a rise in serum corticosterone levels in hypophysectomized mice corresponds with spleen cell production of irACTH. Furthermore, preliminary experiments showed that B cell depletion blocked the normal rise in serum corticosterone levels after herpes simplex virus infection of intact mice. It seems that a similar system operates in vivo in humans. Typhoid vaccine. which induces lymphocyte-derived irACTH production in vitro, caused a time-dependent increase in the number of irACTH-positive lymphocytes in both hypopituitarism and normal short children. A rise in serum cortisol levels was seen in one patient with hypopituitarism and all normal patients. The above regulatory circuit also seems able to act in the reverse direction. Pituitary ACTH and α-endorphin can behave like lymphokines by being immunosuppressive at 0.5 μM in an in vitro antibody synthesis system. Further, lymphocytes were shown to have high-affinity receptors for both of these hormones. Thus, it appears that the immune and neuroendocrine systems have the ability to signal each other through common or related peptide hormones and receptors.