The factor responsible for the prolonged survival of skin grafts taken from mice bearing Ehrlich ascites tumor (EAT), was investigated in this study. It could be demonstrated that the prolonged survival is not confined only to the grafts taken from mice injected previously with full EAT. Grafts taken from mice inoculated with EAT cells separated from the supernatant by centrifugation, or from mice inoculated with the supernatant only, has also a prolonged survival. Treatment of EAT with ultrasonics prior to its inoculation to the donor, was followed by a shorter survival of the graft though this was still longer than that in the control group.