Abstract: Substantially spherical polymer particles containing a colloidally, stably and uniformly dispersed magnetically responsive substance of about 5 nm to about 500 nm in diameter provide a superparamagnetic reagent carrier. Preferably, the polymer particles have a hydrated diameter of about 0.5 to 100 um, the polymer is a non-ionic cross-linked polyacrylamide gel and the magnetically responsive substance is iron oxide granules. The magnetically responsive substance may be present in an amount so that the polymer particles have a specific gravity of between 1.2 and 2.7. The reagent carrier is prepared by swelling polymer particles in a solution of iron salt so that substantially all of the iron salt solution is taken upon by the polymer particles and then converting the iron salt in situ to insoluble colloidally dispersed iron oxide granules.