To sum up briefly, the data accumulated during the past decade show conclusively that plants infected with certain virus diseases develop a defense reaction beyond the natural immunity inherent in the plant. There are some virus diseases from which plants recover and become refractory to reinfection. The immunity that ensues is of the carrier type, since the plant continues to harbor the virus. In many instances-in all instances so far studied-plant cells infected with one strain of a virus become immune from infection with a closely related strain. In this case, the immunity is usually of the chronic-disease type, since the plants lose neither the virus nor the symptoms produced by the virus. In both types of immunity, the nature of the defense mechanism is unknown. The nature of the mechanism is of importance not only with respect to plant virus diseases but also in the general field of immunity as a whole, for even in animal diseases the exact nature of the defense reaction is by no means solved. It is to be hoped that further studies on plant virus immunity will at least indicate the byways along which we need to go in pursuit of this elusive question.