A commercially available preparation of corticotropin was found to have potent cytotoxic activity for several established cell lines. Neither synthetic corticotropin nor alpha melanocyte stimulating hormone demonstrated this cytotoxic activity. Gel filtration allowed separation of a 2100 dalton cytotoxic peptide from the actual corticotropin present in the commercially prepared material. The structural relatedness of the cytotoxic peptide to corticotropin was demonstrated by neutralization with antisera to alpha melanocyte stimulating hormone. These studies indicate the existence of a newly identified ACTH related peptide with cytotoxic activity. © 1983.