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Fluids with ferromagnetic properties may be formed by a colloidal suspension of solid magnetic particles such as magnetite in a parent liquid. The viscosity of the fluid in a magnetic field is predicted by dimensional analysis to be a function of the ratio of hydrodynamic stress to magnetic stress, γν 0 MH. The prediction is verified experimentally and from the correlated data it is determined that for values of this ratie greater than 10− 4 there is no change of viscosity in an applied field. For values between 10− 6 and 10− 4 the viscosity is a function of both magnetic field and rate of shear. Theoretically, for high values of applied field, the viscosity again becomes independent of field or shear rate.