SUMMARYThe mechanism of active immunological enhancement of skin allografts was studied in CBA/J mice immunized with lyophilized spleen tissues from A mice by means of passive and adoptive transfer of immunity. The results of adoptive transfer of immunity revealed the significance of inhibition of the cell-mediated response in actively immunized mice. Hydrocortisone injected at an appropriate time led to a further significant prolongation of skin allograft survival in both the actively and passively immunized animals.