1. A reliable method of parathyroid grafting in rats, and of assessing the functional state of such grafts in terms of blood calcium has been devised. 2. Parathyroid homografts between rats of proven genetic diversity may be rejected as promptly as skin; but the majority survive for an indefinite period, there being no intermediate class. 3. Surviving parathyroid grafts in this genetically diverse combination invariably succumb in the presence of the immunity induced by a later skin graft from the same donor. 4. Parathyroid grafts between rats of less genetic diversity have usually responded to skin graft-induced immunity in the same manner as those between more dissimilar rats. The less divergent group, however, revealed an occasional persistent functioning of parathyroid grafts even in the face of the sensitivity incidental to an accelerated reaction to skin. 5. The hypothesis is presented that the parathyroid tissue of the rats studied was deficient in effective transplantation antigens as compared with those of their skin. This weakness in apparent antigenicity may be a reflection of a deficiency in production, availability, or content of antigens in the tissue transferred.