Sequential sections were prepared from isografted or allografted sciatic nerves in adult female DBA/2 mice and were compared with the wallerian degeneration in the same inbred strain. Transverse sections were studied by qualitative and quantitative histology. Rejection was manifested by an important infiltration of mononuclear cells, including large pyroni-nophilic cells, around the grafted nerve, followed by the appearance of irregular infiltrates around the neovessels within the nerve. In parallel, the number of endoneural nuclei increased less rapidly in allografts than in isografts up to day 15 and diminished thereafter, in association with a possible reduction of neural vascularisation.The results of nerve grafting in clinical cases are extremely difficult to assess because of the various origins of the grafts, the diversity of the preservation techniques, and the varying standards adopted for estimating recovery. Although peripheral nerves are quite likely to elicit an immunological rejection reaction when transplanted into an allogeneic host, as do most other tissues, very little is yet known of the mechanisms and functional or histological manifestations of this reaction. Hence, despite many attempts at conditioning peripheral nerves to minimise immunological rejection, the distinction between technical and immunological failures often remains impossible (1–4). A better knowledge of the rejection of peripheral nerves appears critical if the recent developments of transplantation immunology (e.g., tissue typing, host conditioning) can be applied to nerve grafting in clinical situations. The major difficulty in analysing the histological changes associated with nerve allograft rejection is the presence of the wallerian degeneration reaction of the graft which consists of a proliferative process of different kinds of cells including, in addition to Schwann cells, macrophages, and fibrocytes (5, 6).We report here the sequential study of tissue sections of sciatic nerves grafted into inbred allogeneic mice compared with syngeneic grafts performed in the same recipient. Both allografts and isografts were compared with the wallerian degeneration obtained by section of the sciatic nerve in mice of the same inbred strain. The histology of the immunological rejection reaction could therefore be defined on quantitative grounds.