A device for the preliminary treatment of samples immediately prior to flow cytofluorimetric analysis is described. The device is intended for several procedures: (a) mixing of batched sample volumes with the reagent and efficient stirring of the mixture; (b) disintegration of cell aggregates; and (c) disruption of cell membranes to release the cell contents (chromosomes, micronuclei, nuclei etc.). The pretreatment is useful for studying the kinetic parameters of fast cellular processes in the flow, a more correct analysis of the cell cycle and the study of karyotypes of single mitotic cells. The device was called a magnetic microstirrer.