The shear stress tau-shear strain gamma behavior of suspensions of carbonyl iron particles in silicone oil was determined at a shear rate gamma = 0.042 s(-1) as a function of applied magnetic induction B-0 (0.5-6.0 kG), volume fraction of particles phi(0.05-0.30), and roughness of the shearing surfaces. At low fields, the steady-state shear stress tau(S) was significantly higher for a roughened surface compared to a smooth surface, but became only slightly dependent on roughness at high fields. In accord with theoretical considerations, the ratio tau(S)/phi was proportional to the square of the calculated magnetic field H-0 inside the suspension at low fields, but its dependence on H-0 became smaller as the field increased. The smaller dependence may result from the magnetic saturation of the particles or the slipping boundary condition.