Not only has the primary structure of porcine (thyro)calcitonin (CT) been elucidated but also the synthesis of the hormone has been reported (Rittel, Brugger, Kamber, Riniker & Sieber, 1968). Radioimmunoassays have been described by Deftos, Lee & Potts (1968), Arnaud, Tsao & Kaplan (1968) and Tashjian & Melvin (1968). The purpose of this communication is to report on the application of a highly sensitive radioimmunoassay to the study of the site of immunological activity within the porcine CT molecule and the extent of cross-reactivity between human and porcine CT. A partially purified CT preparation from porcine thyroid glands, which had a biological activity of 8 MRC units/mg., was used for immunization. Five rabbits received an i.m. injection of 0·2 mg. of this material at 4-week intervals. The hormone was adsorbed on to aluminium hydroxide gel (Superfos Exp. Co, Copenhagen) and 0·5 ml. of the mixture (containing about 8 mg. Al(OH)