The abundance and cytoplasmic organization of keratin filaments enables them to contribute to the maintenance of structural integrity in epithelial tissues. Co-polymers of the type II keratin 8 and type I keratin 18 form the major intermediate filament network in simple epithelia. We investigated the mechanical properties of K8-K18 filament suspensions using rheological assays in conjunction with light and electron microscopy. Suspensions of K8-K18 filaments behave like a viscoelastic solid under standard assembly conditions. Bulk elasticity is weakly dependent on deformation frequency but is very sensitive to the concentration (G' similar to C-1.5) and size of individual keratin polymers, in agreement with recent models of semiflexible-polymer physics. K8-K18 filaments can self-organize to form a bundled network that exhibits gel-like mechanical properties. In all cases the mechanical properties of the suspensions correlate with the structural features of individual polymers, as seen under light and electron microscopy. Importantly, these bulk viscoelastic properties of K8-K18 filaments are revealed only when interfacial elastic effects are minimized by the application of phospholipids at the air liquid interface. Suspensions of K5-K14 and vimentin filaments also exhibit interfacial elasticity, which distorts the interpretation of the viscoelastic moduli as determined by standard rheometry. The potential for modulation of mechanical properties through self-organization may be a general property of keratin polymers and contribute to their organization and function in vivo. (C) 2003 Elsevier Science (USA). All rights reserved.