The nonlinear mechanical properties of cells in response to large forces as they arise under physiological conditions are not well understood. In this work, a magnetic tweezers setup optimized for large forces was developed. Forces of more than 100 nN can be applied, which has not been achieved previously with a setup of this type. The time- and force-dependent nonlinear material properties of various cell types were examined using this setup. The creep response always followed a power law, as in the linear case. With increasing force, stress stiffening and shear fluidization were observed....