Morphology of intracytoplasmic nucleocapsid measles virus from mouse brain and tissue culture was studied with the use of ruthenium red by tilting and high resolution electron microscopy and compared with the paramyxovirus-like tubules found in multiple sclerosis (MS). Both in vivo and in vitro the measles nucleo-protein profiles were surrounded by 'fuzzy' material which could be resolved in a pentagon shape and this stained specifically with ruthenium red. The tubules found in MS appeared not to have a 'fuzzy' coat and also did not stain with ruthenium red. The main difference observed between infected tissue culture cells and mouse brain was that in the latter no alignment of measles nucleoprotein was observed under the cell membrane and no budding particles were seen. The virus could not be passed to mice or tissue culture from the infected mouse brain.