Immunological phenotyping of acute leukaemias is important for a more precise diagnosis with respect to both cell lineage and maturation level. We have developed a rapid and reliable method for immunophenotyping, based on the use of magnetic monodisperse beads coated with monoclonal antibodies. After only a 10-min incubation of immunomagnetic beads (IMB) with mononuclear cells isolated from bone marrow or peripheral blood, the percentage of rosetting cells can be counted in the microscope. A panel of 16 monoclonal antibodies against haematopoietic cell-surface antigens was applied on 29 cases of acute myelogenic (AML) or lymphocytic (ALL) leukaemias, in order to compare immunological typing by immunomagnetic beads with immunofluorescence staining (IF). In all the cases tested, the two methods showed a virtually identical antigen distribution. The procedure described offers the advantages of being fast and simple to perform. Moreover, it has a high specificity and is easy to interpret in cases with low antigen expression.