In this work, the magnetorheological behavior of concentrated (10% solid volume fraction) iron-based magnetorheological fluids was studied. Two different compounds were added to stabilize the suspensions against aggregation and settling processes: A surfactant (aluminum stearate, AlSt), and a gel-forming agent (silica nanoparticles). The shear stress vs. shear rate flow curves of the suspensions were obtained in a wide range of applied magnetic fields with the aim of determining the intensity of the field-induced yield stress, that is, the strength of the so-called magnetorheological (MR) effect. The suspension stabilized against particle aggregation by surfactant addition reached the strongest MR response, while the opposite behavior (the weakest MR response) was found in the suspension that contained silica nanoparticles added as anti-settling agent. The scaling between the yield stress and the magnetic field strength (sigma(y) similar to H-n) was calculated and compared with the predictions of theoretical structural models. The results demonstrated that such scaling depends on both the aggregation degree of iron particles in the suspensions and the ratio H/M-S0 (M-S0, saturation magnetization of the iron powder), showing a continuous decrease in the exponent n for high enough magnetic fields. (C) 2008 The Society of Rheology.