ACTH (4-11) is a peptide fragment of adrenocorticotropic hormone (ACTH; ), a peptide hormone found in the brain that is involved in the biological stress response. ACTH (4-11) induces differentiation of melanocytes when used at a concentration of 1,000 nM. It inhibits angiotensin converting enzyme (ACE) activity in homogenized canine lung with an IC50 value of 24 μM. ACTH (4-11) has been used to determine substrate specificity of recombinant H. zea carboxypeptidase B enzyme (CPBHz).