The biochemical and functional characterization, and the regulation of plasma membrane expression of the leucocyte tyrosine phosphatase CD45, have been investigated in neutrophils from healthy donors and patients undergoing haemodialysis. CD45 proteins of 180 kD and 130-150 kD were precipitated from neutrophils from both healthy subjects and haemodialysed patients. Prolonged storing, as well as trypsin treatment of samples containing the 180-kD CD45 protein, generated the 130-150-kD polypeptides. The 130-150-kD CD45 polypeptides carried extracellular CD45 epitopes, including the sialic acid-related UCHL1 epitope (CD45RO). Furthermore, these trypsin-generated CD45 polypeptides did not possess phosphatase activity, which could be detected on the 180-kD protein. A remarkable quantitative increase of cell surface expression of the neutrophil CD45 components was detected both after in vitro neutrophil activation and after dialysis treatment with neutropenic membranes. The CD45 biochemical pattern did not qualitatively change upon either in vitro or in vivo dialysis-induced neutrophil activation. The upregulated expression of CD45 on neutrophils from dialysed patients correlated with the neutropenic effect induced by the different dialyser membranes. Maximal upregulation of CD45 expression was observed after 15 min of dialysis with neutropenic membranes, and normal expression levels were restored after 1 h. By contrast, increase of CD45 plasma membrane expression induced in vitro by treatment of normal neutrophils with the degranulatory agents fMLP or Ca2+ ionophore was maintained. These results demonstrate that neutrophil cell surface expression of the 180-kD CD45 protein is upregulated during the in vivo haemodialysis process, and suggest that a proteolytic activity could regulate the enzymatic activity of CD45 by degranulation of its cytoplasmic phosphatase domains.