The effect of particle interactions on sedimentation field-flow fractionation of a binary suspension is studied on the basis of a phenomenological model. The solvent fluid is regarded as a background medium mediating the effective interactions between solute particles. A binary suspension with large and small particles is considered. Equilibrium profiles of both species are calculated from an approximate expression for the Helmholtz free energy of a hard sphere mixture. The effect of solute particles on suspension viscosity is taken into account via a semi-empirical relationship. It is found that retention factors of both species depend strongly on the two mean volume fractions. The ratio of the two retention factors is nearly independent of the concentration of the large particles and first decreases, but then increases significantly with the concentration of the small ones. (C) 2003 American Institute of Physics.