Static yield stresses are measured on magnetic suspensions as a function of the applied magnetic field. Two differents systems are used: One is an aqueous suspension of polystyrene spheres containing iron oxide inclusions and the second one is of ordinary polystyrene spheres dispersed in a ferrofluid. The experimental results are analyzed with the help of a model based on the calculation of forces between two particles in an effective medium. This model is discussed and tested against experimental measurements of forces on centimetric iron spheres placed in a magnetic field.