The human lymphoblastoid leukemic cell line (CCRF-CEM) was induced to differentiate with phorbol 12-myristate 13-acetate (PMA). During differentiation, assessed by monitoring the cluster of differentiation (CD) profile, the prosome (proteasomes, multi-catalytic proteinase) distribution and composition were studied by microscopy, flow cytometry and Western blot analysis. Changes in prosome subunits were monitored using 3 monoclonal antibodies anti-p23K, p29K and p31K. There were changes in the subcellular distribution of prosome antigens in PMA treated cells compared to untreated cells. The amount of cytoplasmic prosomal antigens decreased during the first three days of differentiation and the membrane antigens increased; meanwhile there was an increase of p53 and no change in actin protein levels. As mitotic cyclins are degraded by the ubiquitin pathway and therefore via the prosome, the decrease observed in differentiated cells suggests that prosomes are involved in the cell cycle and thus in cell proliferation.