Intraperitoneal injection of serum from a patient with the Guillain-Barré syndrome (GBS) produced GBS-like signs in mice: inadequate respiration and weakness in the legs. We studied the clinical, electrophysiological and pathological features of these mice. Three groups of three mice were injected with patient serum from days 6, 10 and 15 after onset of neurological symptoms. GBS-like signs in mice were observed only with serum from day 6 and improved within 48 h. When serum was frozen and thawed more than once no signs were seen. Electrophysiological measurements of the sciatic nerves of injected and control mice were done before and after serum injection. Five days after injection of patient serum of day 6, the mice showed a significant decrease in the ratio between CMAP amplitude from proximal and distal stimulation and increase in H-M interval from proximal stimulation. These electrophysiological changes returned to normal within 12 days. The sciatic nerve showed no morphological abnormalities. Our results indicate that the observed GBS-like signs in mice are caused by peripheral nerve dysfunction.