Research on cytoskeletal elements of eukaryotic cells has been expand­ ing explosively during the past 5 to 10 years. Due largely to the employment of electron and immunofluorescent microscopy, significant results have been obtained which have provided interesting new insights into the dynamics of nucleated cells at the structural, physiological, as well as developmental levels. While a substantial amount of knowledge has accumulated on the function of microfilaments and microtubules, the roles of the third major class of cytoskeletal structures in vertebrate cells, the intermediate filaments, have largely resisted clarification. The investigation of cultured cells and of tissues from various developmental stages has furnished a host of information on the inter-and intracellular distribution of the different types of intermediate filaments and led to the contention that they have a structural and organizing function in the cytoplasm of vertebrate cells. However, the results of recent experimen­ that vertebrate cells can function perfectly in the tation have shown complete absence of cytoplasmically extended intermediate filament meshworks. It is legitimate to suppose, therefore, that their function in vertebrate cells is much more subtle and complex than generally presumed. Our interest in the structure and function of intermediate filament proteins was initiated approximately 7 years ago while working on the regulation of macromolecular synthesis in picornavirus-infected mam­ malian cells. In attempts to demonstrate virus-induced changes in the nuclear protein components of the host cells, the nonionic detergent extraction method was used to purify nuclei.