1. The resistance against toxic products of typhoid bacilli of the heart tissues of typhoid-immune animals is greater than that of nonimmunized animals. 2. The acquired increase of resisting power is specific. 3. The increased resistance is not caused by antitoxin, but by a specific biological alteration of tissue cells. 4. It is maintained that permanent acquired immunity, at least in typhoid, instead of being due principally to antibodies, is caused by a lasting specific increase of resisting power acquired by the tissue cells against the toxic products of bacteria. 5. This theory seems to hold good with other infectious diseases in which one attack conveys permanent immunity.