Histological studies of interlamellar keratoplasty in different inbred strains of rats (CAP, LEW) without additional sensitization--i.e. first-set reactions--are described. The corneas of 65 eyes, after allogeneic or syngeneic grafting, were examined from the 3rd to the 90th day--every 2nd day at the beginning of the reaction and later every 5th day. After syngeneic grafting (LEW leads to LEW) a non-specific healing-reaction (only slight vascularization of the graft bed, edema, granulocytic infiltration) reached its climax on the 6th day and subsided by the 10th day. After allogeneic grafting (CAP leads to LEW; RtH-1-incompatible) the non-specific-healing reaction progressed into a second phase, namely the specific reaction: increasing infiltration of the host cornea and the graft with small lymphocytes, blast cells and macrophages, directly followed by severe vascularization, reaching its climax about the 14th day. A third, phagocytic phase succeeded the infiltration leading to elimination of the donor cells, but leaving the donor stroma undamaged. All these alterations had almost completely disappeared after 35 days. Thus, the corneal allograft reaction is discussed as a typical immunological reaction leading to the destruction of transplantation-antigen-bearing cells and permitting observation of the different reaction phases more clearly than in most other tissues.