A medical, surgical, or operating kit for occluding or controlling blood flow or fluid flow is disclosed. The kit is comprised of (a) a dispersion of magnetic microparticles which can be administered into a blood vessel or fluid vessel, and (b) an external magnetic field source which is adapted to be placed at the desired site above the path of the bloodstream or fluid flow to form a plug or blockade within the blood vessel or fluid vessel that occludes or controls blood flow or fluid flow.