The diagnostic intracytoplasmic perinuclear inclusion bodies within the fibroblastic tumor cells of recurring digital fibrous tumor of childhood (Reye tumor) were found to be ultrastructurally composed of condensed microfilaments that were continuous with axially oriented cytoplasmic filament bundles running toward the cell membrane. Immunofluorescence microscopy performed with antibodies raised against vimentin and against actin revealed a strong positive reaction for vimentin in the tumor cell cytoplasm, whereas the spherical inclusion bodies were actin-positive. From these results the following conclusions may be drawn: (I) the mesenchymal origin of the Reye tumor cells is consistent with the identification of vimentin intermediate filaments and (II) the actin-positive spherical inclusion bodies appear to represent pathological aggregations of microfilaments.