The present invention concern a method and a device for the isolation of non-magnetic cells from a heterogeneous sample solution containing biological material including desired and undesired cells. The method comprises the steps of: —adding magnetic or magnetizable particles to the sample, wherein said particles have sizes in a range from 100 nm to 4 μm and exhibit surface components which support specific association with target cells, wherein said target cells comprise are either said desired or said undesired cells; —decreasing said external magnetic field gradient; —incubating said sample solution with said magnetic particles to obtain a magnetized cell fraction; —washing said magnetized cell fraction using a washing solution to reduce non-specific binding; —increasing said external magnetic field gradient; —separating said magnetized cell fractionation of target cells from said sample; wherein said sample solution is subjected to an external magnetic field gradient throughout said adding, incubating, washing and separating steps, and wherein said sample solution is rotated at least during said adding, incubating and washing steps.