A pressure-driven apparatus was used to measure the yield stress of magnetorheological (MR) fluids as a function of the applied magnetic field, the carbonyl iron particle content, and the amount of surfactant used to stabilize the MR fluid. The yield stress was measured from the pressure difference necessary to initiate flow of a MR fluid in a straight tube present in a magnetic field. Yield stress measurements were made on MR fluids that contained up to 30 vol % carbonyl iron particles, up to 14 vol % surfactant oleic acid, and the remainder 100 cS silicone oil. In the absence of an applied magnetic field, the MR fluids did not have a yield stress and behaved as Newtonian fluids. In the presence of magnetic fields up to 2.2 kG, the MR fluids had yield stresses up to 1.9 kPa. An effect of the tube size on the yield stress was observed-for MR fluids above 15 vol % carbonyl iron and magnetic fields above 1.0 kG.