Ultrastructural characteristics of the early steps of PML virus infection, i.e. adsorption, penetration, and intracellular virus transport were demonstrated in astrocytes. These phenomena were more frequently seen in the cytoplasm than virus assembly in the nuclei. Infection of the astrocytes is usually non-productive. Light- and electron-microscopic analysis of the present case and 23 cases published in the literature since 1970 leads to the conclusion that the relation is inverse between the appearance of giant, bizarre astrocytes and that of the productive infection of the non-transformed one. The transformation of the astrocytes seems to be induced by infection with the PML virus.