Magnetophoretic mobility arises from the motion of an electrically neutral body in a viscous medium when exposed to an inhomogenous magnetic field. Conforming to other types of mobility, which generally have the form of a "reciprocal friction coefficient" (such as electrophoretic mobility), we have defined the magnetophoretic mobility, m, as a ratio of a particle-field interaction parameter, phi(m), to the particle friction coefficient, f, so that m = phi(m)/f, with the parameter phi(m) equal to the product of the magnetic susceptibility of the particle relative to that of the medium, Deltachi, and its volume, V-m, so that phi(m) = DeltachiV(m). The particle mobility is an important factor in predicting separation when a mixture of particles of different mobilities is exposed to an external field. We have combined the information about the particle magnetophoretic mobility, measured by cell tracking velocimetry, and the theory of the split-flow thin channel fractionation, to describe particle and cell separations in a flowing solution through an annular channel coaxial with a quadrupole magnetic field. The theoretical model was verified by experiments with monodisperse magnetic polymeric particles and human white blood cells.