Ethylene oxide gas is used to sterilize plastic medical equipment including capillary flow dialysis membranes. To test whether ethylene oxide retained in the dialyzers might be neurotoxic, tissue culture medium was incubated in the blood compartment of dialyzers. Embryonic rat dorsal root ganglion neurons were then incubated in this medium for up to 5 days. During the first 24 hours axonal growth was normal. During the next 24-hour period varicosities appeared on axons, and after 4 days neuron cell bodies died. The pattern of degeneration was identical to that observed when cultures were exposed to an atmosphere containing 1 ppm ethylene oxide gas. Culture medium introduced into dialyzers after routine prerinsing still caused degeneration, which was not completely abolished even by a 10-liter rinse. When medium was exposed to identical dialyzers sterilized by gamma irradiation, no changes were seen in culture. Identical morphological changes were produced by using dialysis patient serum in the culture medium in place of the usual calf bovine serum. Such changes were never seen with control human serum. Since ethylene oxide is toxic to the human peripheral nervous system, it is proposed that ethylene oxide in dialyzers may contribute to the progressive neuropathy observed in patients on long-term hemodialysis.