Abstract When the mature sperm of Saturniid moths are ejaculated during copulation, they are mixed in the male's common duct with a secretion which induces vigorous motility in the sperm. The active substance in this secretion is soluble in 7·5% trichloroacetic acid but not in organic solvents; it is stable in aqueous solution at 100°C, but is only partially dialysable. These properties plus the fact that activity is completely destroyed by proteolytic enzymes suggest that the substance is a polypeptide. When filtered through Sephadex columns, all of the activity can be resolved into a single peak corresponding to a molecular weight between 1600 and 4500, most probably about 3100. Even in highly active preparations this peak could not be detected with ninhydrin reagent. Calculations indicate that the lowest concentration of activator that will induce motility is not more than 10 −8 m. Most naturally occurring substances having properties similar to those of the activator have proved to be hormones.