The effect of serum from a patient with acute motor axonal neuropathy (AMAN) on cultured motor neurons was studied. The ventral spinal ventral tissue was isolated from embryonic rats and digested into dissociated cell suspension for culture in vitro. The cultured cells were stained with SMI-32, a non-phosphorylated neurofilment marker monoclonal antibody to identify motor neurons. The 6 days' cultured cells were exposed to the AMAN patient serum in a concentration of 25%, and to the normal human serum as the control. Positive PennerO:19 Campylobacter jejuni lipopolysaccharide antibody in the AMAN serum used in this experiment had been testified. The serum-cultured motor neurons were observed morphologically and also stained by Guillery Shirra and Webster method. With this staining, degenerated nerve fibers were brown-black and normal nerve fibers were brown-yellow. At the 9th h after the AMAN serum exposure, the axon degenerated and was stained brown-black due to increased silver-phile property. At the 12th h, the neuron soma began to swell and nuclear deviation with silver granules depositing in the cytoplasm. At last, the neurons began to die from the 16th h of the exposure. However, the control motor neurons did not show these alterations in the same period of culture. The serum of AMAN patient may be toxic to the neurite of motor neuron and thus cause axon degeneration, then soma alterations and death followed. It is suggested that Campylobacter jejuni lipopolysaccharide antibody may play an important role in this process without the participation of macrophages and complements.