IF are major cytoskeletal and karyoskeletal components of eukaryotic cells (Steinert and Roop 1988). In numerous instances, their constituent protein subunits have been shown to be substrates for a variety of kinases such as A-kinase, C-kinase, and Ca++/calmodulin kinase (Geisler and Weber 1988; Inagaki et. 1988; Ando et al. 1991), as well as p34cdc2 (Chou et al. 1990; Peter et al. 1990; Ward and Kirschner 1990; Dessev et al. 1991). To date, all of the phosphorylation sites that have been mapped are in the non-alpha-helical amino- or carboxy-terminal domains (Steinert 1988; Ando et al. 1989, 1991; Geisler et al. 1989; Chou et al. 1991), and these secondary modifications can lead to IF reorganization and/or disassembly in vivo and in vitro (see, e.g., Iganaki et al. 1988; Lamb et al. 1989; Chou et al. 1990; Peter et al. 1990; Heald and McKeon 1990; Dessev et al. 1991). In addition, it is possible that the exchange seen between subunits and polymerized IF in interphase following the microinjection of unpolymerized protein (Vikstrom et al. 1989; Miller et al. 1991) may also be regulated in some fashion by phosphorylation/dephosphorylation reactions. In cultured fibroblasts such as BHK-21, the interphase equilibrium state that favors IF polymerization is shifted dramatically to a disassembled state in mitosis, apparently due to enhanced phosphorylation at specific sites mediated through the activity of p34cdc2. However, in other cells in mitosis, such as HeLa, the mechanisms involved in the regulation of cytoskeletal IF remain unclear. Therefore, no one common mechanism appears to be responsible for IF regulation during cell division. On the basis of the majority of data available, it appears that the regulation of IF phosphorylation plays an important role in the regulation of the supramolecular organization of IF cytoskeletal and karyoskeletal networks, especially in the remodeling events that take place as cells enter and exit mitosis. Although the functional significance of IF phosphorylation during interphase is not as obvious as it is in some mitotic cells, we are tempted to speculate that there may be a connection with mechanisms involved in signal transduction, since IF proteins appear to be targets for kinases known to be activated by second messengers such as Ca++ and cAMP.