Method of separation employing magnetic particles and second medium

A method for separating cells from a mixture containing said cells and other components, which method comprises:

• combining in an aqueous medium said cells and said other components, and superparamagnetic or paramagnetic particles having immobilized thereon a member of a specific binding pair (sbp) that specifically binds to said cells to form a complex of said cells and said superparamagnetic or paramagnetic particles (cell-particles),
• layering said aqueous medium with a second liquid medium that is of different density and/or viscosity than said aqueous medium such that said aqueous medium is layered above said second medium,
• subjecting said aqueous medium and said second liquid medium to a magnetic field gradient sufficient to cause said cell-particles to migrate into said second medium and to be washed thereby, and
• separating said cell-particles from said second medium.

An assay method for the determination of a particulate biological material (PBM) that is an analyte, in a sample suspected of containing said PBM and other components, said method comprising;

• (a) combining in a first liquid medium,
• said sample,
• magnetic particles having immobilized thereon a member of a specific binding pair (sbp) that specifically binds to said PBM to form a complex of said PBM and said magnetic particles (PBMMP),
• a labeled sbp member that specifically binds to said PBM, such that said labeled sbp member can be separated from said first liquid medium in relation to the concentration of said PBM in said sample;
• (b) layering said first liquid medium with a second liquid medium that is of different density and/or viscosity than said first medium, such that said first medium is layered above said second medium;
• (c) subjecting said first liquid medium to a magnetic field gradient whereby said PBMMP migrates into said second liquid medium and is washed thereby;
• (d) separating said PBMMP from said second liquid medium;
• (e) examining said second medium, said separated PBMMP or said PBM for the presence or amount of said labeled sbp member, the presence or amount thereof indicating the presence or amount of said PBM in said sample.