At the November meeting of the New York Pathological Society, I reported briefly on a series of experiments I have undertaken on white rats, with the aim to investigate whether the great success attained lately in the transplantation of malignant growths from one rat into another is not due, in part at least, to the different behavior of the organism of the white rat to tissue implantation, as compared with other laboratory animals. The majority of the workers who have experimented with the tumors of the white mouse or rat seem to be of the opinion that the success in the transplantations of these tumors is due to the great intrinsic power of limitless proliferation of the cells of these tumors, and they ignore the possible cellular reaction of the organism of the host to the implanted tissue. But the fact that pieces of the same tumor from the same rat or mouse grow readily on white mice of one race, and not at all on the white mice of another race; the fact, further, that while the original tumor, when implanted, grows in only about 10 per cent. of animals used, and, when re-implanted, it grows in 90 per cent., and some other similar instances, seem to indicate, a priori, that the white mouse or rat reacts differently to implantation of tumor, as well as of normal tissue, than the other laboratory animals. My first set of experiments consisted in the implantation of normal tissue from one animal into another. Pieces of skin, liver, spleen, testicle, or mammary gland are implanted under the skin or in the peritoneal cavity of another animal, In the great majority of the experiments (about 60 to 75 per cent.) the pieces remained unabsorbed for as long as three weeks, even when, as in the peritoneum, they do not become attached anywhere, but float like a foreign body.