Abstract Primary demyelination has been documented in organotypic cultures of mouse spinal cord exposed to 25% myelinotoxic sera from 2 patients with multiple sclerosis (MS). Between 10% and 90% of the original population of myelinated axons were involved. Younger cultures were more severely affected than older ones. Myelin degeneration was seen to involve the transformation of the regular 12 nm periodicity into smudged areas with an amorphous or finely-lamellated structure. The myelinolysis was accompanied by an active overgrowth of oedematous astrocytic processes which surrounded affected fibres and phagocytosed the degenerating myelin. This progressed until naked axons, invested by swollen astrocytic cytoplasm, remained. A number of oligodendrocytes in each MS serum-exposed culture underwent either acute or subacute destruction. The former occurred during the first few hours of exposure. The latter began after about 4 hr and was completed by 72 hr. Affected oligodendrocytes were phagocytosed following their ensheathment by astrocytic processes. Neurons and astrocytes did not degenerate. The myelin and oligodendrocyte lesions produced by MS serum in vitro are essentially identical to those effected by EAE serum from animals inoculated with whole white matter, except that the phenomena are slower and less widespread. The findings have been compared with patterns seen in human MS plaques and in animal demyelinating conditions.