The toxicity of eosinophils towards peripheral nerves was examined using cultures of posterior ganglion cells from chick embryos. Eosinophils were also derived from a patient with significant "hyper-eosinophilia", peripheral neuropathy, bronchial asthma, myocardial damage and skin involvement. A sural nerve biopsy showed a severe to almost complete loss of myelinated fibers without pathological changes suggesting necrotizing vasculitis. Surviving neurons in a culture containing the patient's eosinophils or serum were markedly decreased in number as compared with those in normal subjects. These results support the possibility that the peripheral neuropathy in the patient caused by toxic agents derived from eosinophils.