Myelinated cultures of mouse spinal cord were exposed to sera obtained from rabbits affected by experimental allergic encephalomyelitis following challenge with whole white matter in complete Freund's adjuvant. In the presence of complement, the tissue response begins with an increased birefringence of its myelin sheaths. This is rapidly followed by a gamut of changes leading to demyelination. This study reports that, in the absence of complement, the response is arrested at the stage of increased birefringence. In this way, this early stage of the demyelinating process was available for detailed examination by light and electron microscopy. The brightened myelin sheaths appeared with a few hours of exposure and were seen around all axons and sometimes around cell bodies. This was often accompanied by abrupt breaks in the sheaths and angularly shaped myelin figures. Examination by electron microscope revealed a uniform increase in the myelin period from 11 nm. to 22 nm. The normally double intraperiod line was increased to four electron-dense leaflets, the additional two appearing to be derived from the close apposition of an additional electron-dense layer on the outer surface of the myelin sheath or oligodendrocytic membrane. Oligondendrocytes responsed with a prolific growth of processes whose membranes compacted to form swollen myelin. Neurons, astrocytes, and neuropil showed no changes. In its early stages, at least, the swelling was reversible. It would appear, therefore, that we have isolated the first stage of antiserum-induced demyelination in vitro, a stage which is now available for further study.