Adrenocorticotropic hormone (ACTH) is the primary regulator of adrenal glucocorticoid production. Elevated levels of ACTH play a critical role in disease progression in several indications, including congenital adrenal hyperplasia and Cushing disease. We have generated a specific, high-affinity, neutralizing monoclonal antibody (ALD1613) to ACTH. In vitro, ALD1613 neutralizes ACTH-induced signaling via all 5 melanocortin receptors and inhibited ACTH-induced cyclic adenosine monophosphate accumulation in a mouse adrenal cell line (Y1). ALD1613 administration to wild-type rats significantly reduced plasma corticosterone levels in a dose-dependent manner. In rodent models with either chronic infusion of ACTH or acute restraint stress-induced ACTH, corticosterone levels were significantly reduced by ALD1613. Administration of ALD1613 to nonhuman primates on days 1 and 7 stably reduced plasma cortisol levels >50% for 57 days. ALD1613 demonstrates the potential of a monoclonal antibody to be an effective therapeutic for conditions with elevated ACTH levels.