In this paper we describe a new, rapid and sensitive method to determine plasma cell isotype and clonality in bone marrow using flowcytometry. With the use of a new fixation and permeabilization reagent (Permeafix), which preserves cell structure and morphology, and a monoclonal antibody (Mab) specific for plasma cells (B-B4), it has become possible to specifically select plasma cells and to determine the cytoplasmatic immunoglobulins by flowcytometry. Thirty successive bone marrow aspirates from multiple myeloma patients and patients with MGUS were studied as well as 10 bone marrow samples from patients with reactive plasmacytosis. Each sample was analysed both by immunofluorescence on cytospin smears and FACS analysis. There were no discrepancies between plasma cell isotype as determined by FACS and cytospin. Moreover, FACS analysis was shown to allow detection of very low numbers of plasma cells and to determine whether these plasma cells are mono- or polyclonal. Possible applications are discussed.