Particle-level simulations are employed to investigate the mechanism by which nonmagnetizable particles enhance the field-induced shear stress in magnetorheological fluids. Large amplitude oscillatory shear reveals that nonmagnetizable spheres increase the suspension stiffness; the transition to nonlinear deformation remains unaffected suggesting that the nonmagnetizable spheres do not alter the stability of the clusters of magnetizable spheres. Snapshots reveal that nonmagnetizable spheres participate in stress transfer via repulsive-force clusters in a mechanism similar to jamming in hard-sphere suspensions. Partial stresses, number of repulsive-force clusters, and transient rheological behavior further support that nonmagnetizable spheres directly enhance the stress via repulsive-force clusters. The repulsive-force clusters contain both magnetizable and nonmagnetizable spheres, which likely explains the observation that nonmagnetizable spheres enhance the magnetic field-induced stress, even though they are not magnetizable. (C) 2017 The Society of Rheology.