Methods for separating, cells, particles, or other molecules of interest (MOI) from unwanted materials not of interest (MNOI). by forced movement of MOI into certain zones having properties which deter the entry of unwanted materials. MOI are tagged with magnetic particles and moved with a magnetic field through a fluid, or zones, of higher specific gravity that prevents, unwanted less dense materials from entering. Buoyant or other forces are used to remove any unbound material from the complexes with ASG before reading. Readable labels include the magnetic particle tagged complex itself, and other labels such as enzymes, chemiluminescent materials, radioactive isotopes, chromogenic and fluorogenic substrates and other labels may be used. The invention applies to many assays, diagnostic tests, separative procedures and chemical syntheses.