Ultrastructural characteristics of adsorption, penetration and intracellular transport of polyomavirus is demonstrated in oligodendrocytes in PML. The findings are compared with those obtained by virological examinations of polyomavirus infections described in cell cultures. The morphology of the eclipse phase and the site of the beginning of virus assembly are also described. The vacuolization of the cytoplasm is considered to be the consequence of the production of viral protein and nucleic acid. The cytoplasmic virions surrounded by membrane in the early and late phases of replication are considered to be “infective virions”, while those in the cytoplasm in a “free” state, aligned along membrane fragments or arranged in crystals are classified as “virus progenies”.

The first demonstration of polyoma virions between the separated lamellae of the myelin sheath of a viable axon is presented; their presence, however, is not considered to play a direct pathogenetic role in demyelination.