Summary Pretreatment of dogs with subcutaneous injections of whole blood from prospective skin donors enhances (facilitates) homograft survival. Control grafts were rejected in 10.3 days while enhanced grafts survived 25.9 days. Once the enhanced graft was rejected the animal was fully sensitized and responded in a normal fashion to second set grafts. Similar injections of foreign blood (not from the skin donor) also prolonged survival but to a lesser extent (mean of 15.5 days). This prolongation was not dependent on the simultaneous presence of an enhanced skin homograft.