As reported earlier, myxoma virus will grow in cultivated tissues from normally resistant species; it has also been found to multiply in homografts of such tissues. Investigation has been made of the factors determining the difference between tissues of resistant species in situ in the animal, and in culture. The issue seems not to be determined by either antibody induction, presence of inhibitors, vascularization, degree of organization, or inability of the virus to penetrate cells. Other possibilities are discussed. The virus survived longest and perhaps multiplied best in tissue grafted away from its normal position and better in homografts than in autografts. In a few experiments, adenovirus type 5 behaved like myxoma. Fibroma virus, on the other hand, failed to multiply in grafts.