Although cryopreservation of human bone marrow has become very common in modem medicine, the knowledge on the effects of this procedure on hemopoietic cells is still limited. We herein have investigated whether the process of concentrating of human bone marrow to its buffy coat, the exposure to the cryoprotectant dimethyl sulfoide (DMSO), and/or the rate of controlled freezing/thawing procedures modifies the flow cytometric analysis of human bone marrow cells. We found that both the exposure of marrow cells to DMSO and/or the freezing procedure significantly modifies both the relative proportions of hemopoietic cell subsets and the intensity of expression of certain surface antigens. Thus,percentages of cells expressing CD7, CD13, CD33 and CD34, were found to be lower in both cryopreserved buffy coat and buffy coat merely exposed to DMSO,in comparison to those in untreated coat samples. Moreover, the intensity of the surface expression of CD33 decreased in cells exposed to DMSO, and further in cryopreserved samples. By contrast, the intensity of the CD19 antigen was higher in the last groups of samples than in the buffy coat or the unfractionated human bone marrow. Our present results suggest that flow-cytometric analysis of cryopreserved human bone marrow cells is not fully equivalent to that corresponding to fresh bone marrow or its fractionated buffy coat.