Abstract 1. 1. The calcitonin activity in extracts of chick ultimobranchial tissue is destroyed by milkd alkaline treatment, digestion with pepsin and oxidation by n-bromosuccinimide, but is relatively stable to treatment with acid. 2. 2. In these properties, and in its molecular weight, chick ultimobranchial calcitonin resembles porcine thyrocalcitonin.