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Key processes in normal and diseased cells depend directly or indirectly on the viscoelastic properties of the cytoplasm. Particle-tracking microrheology is a highly versatile method that measures the viscoelastic properties of cytoplasm directly by tracking fluorescent nanoparticles embedded in the cytoskeleton with high spatial and temporal resolutions. Here we present a new method that combines cell transfection, ballistic injection, and particle-tracking microrheology to monitor changes in cytoplasmic micromechanics following controlled changes in protein expression. We demonstrate that cells transfected with GFP (green fluorescent protein) display viscoelastic properties identical to untransfected fibroblasts, that low levels of expression of GFP-alpha-actinin do not affect cell microrheology, and that the transient transfection with GFP-C3 transferase reduces the elasticity of the cytoplasm of fibroblasts to a similar extent as C3 transferase toxin, which de-activates the GTPase Rho. Combining cell transfection with particle-tracking microrheology opens the way to quantitative, single live-cell mechanical studies where stable cell lines cannot be easily established, but where commonly used transfections can be exploited to manipulate cytoskeletal organization.