Lewis rats (RT1(1] were the recipients of 3-cm nerve grafts from syngeneic Lewis donors or allogeneic ACI (RT1a) donors. Microneurosurgical repair of the nerve graft to the transected sciatic nerve of the recipient animal was performed with 10-0 epineurial sutures. Recipients were randomly allocated to cyclosporin A (CsA) immunosuppressed or untreated groups. Cyclosporin A was administered in the minimal effective dosage to prevent nerve allograft rejection across this major histocompatibility disparity (5 mg/kg per day). Nerve regeneration across the nerve grafts was assessed by sciatic function index (SFI) and toe spread index (TSI) determinations serially and by electrophysiologic, histologic, and morphologic assessments 14 weeks after engraftment. Sciatic nerve regeneration across allogeneic nerve grafts in cyclosporin A immunosuppressed recipients was significantly superior compared to the untreated controls (p less than 0.008) and not significantly different from that across the syngeneic control animals.