Trivalent antimonials will quickly kill the microfilaria of D. immitis without immediately killing the adult worms. The reproductive organs of the females are injured and apparently permanent sterility results. At least some of the sterilized females may die within a few months but some may live for a very long time. Destruction of the microfilaria and their continued absence from the blood stream following the use of the trivalent antimonials cannot alone be accepted as evidence that the adults have been killed. No such effect upon the adult worm has been reported for the arsenicals, cyanines or piperazines. Two of the phenyl arsenoxides, p-arsenosobenzamide and arsenamide, have been shown to kill immediately the adult worms without destruction of the microfilaria at doses which seemed feasible for man. It may require a year or more after the death of the adult worms before the microfilaria disappear.