We have shown that corticotropin (ACTH, α-endorphin, and enkephalins can regulate antibody responses, which suggested a role for neuropeptides in a regulatory circuit between the immune and neuroendocrine systems. ACTH and structurally related peptides were examined here for regulation of mitogen induction of the lymphokine γ-interferon (IFNγ) in C57BL/6 mouse spleen cell cultures. Synthetic ACTH1-39 and a porcine pituitary extract containing ACTH activity were potent suppressors of the IFNγ response. Synthetic ACTH1-39 suppressed the response by approximately 62% at 1 to 3 μM, whereas the porcine extract suppressed by > 90% at 1 to 3 μM ACTH. The greater potency of pituitary extract was shown to be due to the presence of an additional peptide of M(r) 2100 that was reactive with antibodies to the N-terminal region of ACTH (ACTH1-13) possessed potent anti-cellular activity against L cells and various transformed cells, but lacked ACTH biologic activity. The anti-cellular peptide suppressed the IFNγ response by > 99% at 0.05 μM. The ACTH1-39 cleavage products, α-melanocyte stimulating hormone (αMSH; acetylated and amidated ACTH1-13), and corticotropin-like intermediate lobe peptide (CLIP; ACTH18-39) had no effect on IFNγ production. ACTH1-24, like ACTH1-39, has full steroidogenesis activity but also had no effect on IFNγ production, which suggests a dissociation of the immunoregulatory and steroidogenic properties of ACTH1-39. ACTH1-39, and possibly also the anti-cellular 2100 M(r) peptide, is initially synthesized as the precursor polyprotein pro-opiomelanocortin (POMC). Enzymatic processing POMC, first to the active ACTH1-39 or the anti-cellular peptide and then to the inactive smaller peptides, probably plays an important role in regulation of lymphokine and antibody production by ACTH and ACTH-related neuropeptides. This is consistent with the recent demonstration of the production of ACTH-like peptides by lymphocytes.