PURPOSE: Nerve allografts are highly antigenic and, thus, require the continuous use of immunosuppressive drugs. FK 506 was found to pre-vent rejection successfully. However, clinically neurotoxic complications have been noted in the central and peripheral nervous system although an increased rate of axonal regeneration has also been shown after nerve crush experiments. To investigate whether a possible regeneration pro-moting potency of FK 506 is determined via an influence on Schwann cells, Schwann cells were cultured from the sciatic nerve of the rat. METHODS: The effect of 100 micro M FK 506 administered daily on these cultures was assessed microscopically over a period of seven days and compared to an untreated control group of cultures. Additionally, the changes in intracellular calcium were recorded using a laser scanning microscope. In vivo regeneration of autologous rat sciatic nerve grafts was assessed clinically, histologically and morphometrically after two and six weeks. The animals received a daily administration of 0.6 mg FK 506/kg body weight, the control received saline. RESULTS: In vitro FK 506 increased the Schwann cell number in culture significantly compared to non treated cultures, while the fibrocyte population was decreased. FK 506 caused a transient increase of intracellular calcium levels in cultured cells. In vivo, a significantly higher axon count was observed in the FK 506 treated grafts after two weeks regeneration compared with controls. Good regeneration was noted in all grafts after six weeks regeneration. CONCLUSIONS: The increased axon counts and decreased myelin debris in the FK 506 grafts after two weeks indicate an accelerated Wallerian degeneration and increased axon sprouting into the graft initially. The inhibition of calcineurin activity is not the mediator of the neurotrophic effect. FK 506 promotes axonal regeneration through binding to FKBP-12. The increase of intracellular calcium may induce Schwann cell pro-liferation via calmodulin. The therapeutic relevance for autologous nerve grafting, however, has to be defined in further studies.