The contribution of the intermediate filament (IF) network to the mechanical response of cells has so far received little attention, possibly because the assembly and regulation of IFs are not as well understood as that of the actin cytoskeleton or of microtubules. The mechanical role of IFs has been mostly inferred from measurements performed on individual filaments or gels in vitro. In this study we employ atomic force microscopy (AFM) to examine the contribution of vimentin IFs to the nanomechanical properties of living cells under native conditions. To specifically target and modulate the vimentin network, Rat-2 fibroblasts were transfected with GFP-desmin variants. Cells expressing desmin variants were identified by the fluorescence microscopy extension of the AFM instrument. This allowed us to directly compare the nanomechanical response of transfected and untransfected cells at high spatial resolution by means of AFM. Depending on the variant desmin, transfectants were either softer or stiffer than untransfected fibroblasts. Expression of the non-filament forming GFP-DesL345P mutant led to a collapse of the endogenous vimentin network in the perinuclear region that was accompanied by localized stiffening. Correlative confocal microscopy indicates that the expression of desmin variants specifically targets the endogenous vimentin IF network without major rearrangements of other cytoskeletal components. By measuring functional changes caused by IF rearrangements in intact cells, we show that IFs play a crucial role in mechanical behavior not only at large deformations but also in the nanomechanical response of individual cells. (C) 2011 Elsevier Inc. All rights reserved.