Magnetically responsive albumin microspheres can be targeted to the vasculature of specific organs, using extracorporeal magnetic sources. Experiments have been performed on targeting these microspheres to specific regions of normal and tumorous rat tails. This paper quantitatively analyzes the relationship between magnetic forces and the observed microsphere holding. The magnetic forces are determined by the magnetic responsiveness of the microspheres, and by the spatial field of the magnet; both of these are measured. The microsphere holding is defined as that fraction of the microspheres perfusing the tail which are held at a particular site; this is measured at various positions in the tail. The holding as a function of magnetic force is thereby established. To interpret the data, the dynamics of microspheres in blood flow is considered, including motion to a vessel wall, shear forces at the wall, and intersphere attraction. Overall, the method appears favorable for targeting therapeutic drugs to tumor sites in humans.