65 biopsies of peripheral nerve from patients suffering from Guillain-Barré syndrome were studied by electron microscopy. In 48 cases there was macrophagic invasion of the Schwann cells of certain myelinated fibers, and in 32 of these cases some myelin sheaths were stripped away by an elongated macrophagic process. Vesicular disruption of the myelin sheath was observed in only 8 cases and in less than 1% of the myelinated fibers. Uncompacted myelin lamellae were observed in a few myelinated fibers. These ultrastructural lesions are analysed and commented on with a view to selecting patients who are to undergo plasma exchange.