The positions of the half-cystines in the hormone carrier neurophysin are compared with those in three cystine-rich plant proteins. This suggests that neurophysin is a molecule of two domains with each very similar in structure to the domains in wheat germ agglutinin. It is further proposed that in proneurophysin the hormone (oxytocin or vasopressin) is covalently linked to the COOH terminus of neurophysin.