SUMMARYC57BL/6J skin grafts enjoy a prolonged survival on CBA/J recipients, if the C57BL/6J donor is pretreated with high titer recipient antidonor serum. This form of immunological enhancement differs from classical immunological enhancement because the immune serum is administered to the donor rather than the recipient. To distinguish the two, the term “donor enhancement” has been used. Nonspecific immunosuppression of the donor with azathioprine or heterologous or isologous antilymphocyte serum fails to produce a similar prolongation. Moreover, a prolongation is seen even in the presence of a simultaneous ongoing rejection response.