Magnetic separation apparatus and methods for separating colloidal magnetic particles from a non-magnetic test medium in which the magnetic particles are suspended. The separator comprises a container holding the non-magnetic test medium, one or more magnetic wires disposed substantially within the test medium in the container and an external magnet for producing a magnetic field gradient within the test medium. According to the method of the invention, the container holding the test medium is positioned in the separator, producing a magnetic field gradient operative to cause the magnetic particles to be attracted to the areas surrounding the magnetized wires and to adhere to the wires. The non-magnetic test medium is separated from the magnetized particles, which may conveniently be washed while adhered to the wires, and subjected to further analysis, preferably while on the wires. The apparatus and method are useful in separating various target substances from test media by means of substances coated on the magnetic particles which bind specifically to the target substance. A modified buffer solution, when added to the test medium, reduces non-specific binding of the magnetic colloid particles to cells.