Studies of Regression in a Transplantable Tumor in Mice *

In the course of studies8 on the development of transplants of tumor 15091a4 in susceptible and resistant strains of mice, it was noted that after a preliminary period of growth in certain animals the grafts regressed completely. This phenomenon of regression of the transplanted mouse tumor is well known. (See Woglom10 for a review of early work on this subject.) Histologically, necrosis of the graft with great overgrowth of the connective tissue1 or with hemorrhage' has been noted. Woglom,11 who studied regression in the Jensen rat sarcoma, conduded that the tumors were "attacked first at the periphery by some deleterious principle or condition, associated with the connective tissue of the host," resulting in a (czone of necrobiosis followed by what appears to be secondary fibrosis." Little is known of the factors which are responsible for this interesting expression of host resistance. The observations reported in this paper were made in the course of investigations into the nature of these factors.