The fate of microtubules and of vimentin or keratin containing intermediate filaments during infection with fusion or rounding producing strains of herpes simplex virus (HSV) was investigated. Microtubules polymerize early after fusion of cells. However, they do not reconstitute 6-7 hours post infection (p.i.) after release of a colcemid block. Keratin and vimentin are maintained around the original nucleus still inside of recruited cells in the polykaryocyte. Cells of fibroblastic and epithelial origin fuse. Inside of polykaryocytes keratin or vimentin containing fibers seem to polymerize. Keratin is to be found in invaginations in the nuclei surrounded by the inner layer of the nuclear membrane. Anti-keratin antibodies specifically label HSV envelopes located in the cytoplasm or outside of the cell. Controls of the procedure allowed to exclude labelling of HSV envelopes via gpE, which represents HSV induced Fc receptors. Late stages of infected cells contain thickened and condensed keratin fibers. Conversely, vimentin fibers late after infection appear to be evenly distributed and to be thin. Microtubules decay late after infection with rounding producing strains of HSV, whereas keratin and vimentin fibers are still present late after infection.