Abstract B-cell chronic lymphocytic leukemia (B-CLL) is the most common leukemia in adults and is characterized by the accumulation of clonal CD5+ B lymphocytes. Several protein kinase pathways have been claimed to be involved in the regulation of apoptosis and cell survival. We previously demonstrated that Src kinase Lyn is overexpressed at the protein level in leukemic cells as compared to normal B lymphocytes with substantial amount of the kinase anomalously present in the cytosol. Moreover, most of Lyn is constitutively active in resting leukemic cells and is poorly responsive to BCR engagement. The finding that B CLL cells contained cytosolic Lyn fraction and are defective in programmed cell death suggest that the tyrosine phosphorlation of specific cytosolic targets might account, at least in part, for cell resistance to apoptosis. The 75 KDa HS1 protein is one of the major substrate of Lyn kinase upon BCR cross-linking that plays a crucial role in BCR- induced apoptosis in the mouse B lymphoma cell line WEHI-231. A recent study demonstrates that most HS1 protein was constitutively phosphorylated in B CLL patients with poor prognosis whereas only a fraction was phosphorylated in patients with good prognoses. In the present study, the relative HS1 protein levels were measured by Western blot analysis in 50 CLL patients belonging to different clinical stages. The relative HS1 protein levels were compared with corresponding levels in normal peripheral blood and with Jurkat cells. For normal B cells, the mean ± SD for HS1: actin ratio was 0,88 ± 0,10. There was considerable variation in the levels of HS1/actin ratio in CLL cells, which ranged from 0,49 to 2,50. Thus, compared to normal B cells, 15 CLL patients had a HS1 level which fell within the mean ± 1SD HS1 levels for normal B cells, while 9 patients had lower levels and 26 patients had higher levels. When assessed by flow cytometry, HS1 expression was normally distributed among CLL cells in individual patients and the mean levels correlated with those obtained by Western blotting. A difference in the levels of HS1 was also observed between mutated and unmutated patients. Using confocal microscopy and subcellular cell fractionation, we observed that HS1 protein was abnormally distributed in malignant cells as compare with normal B cells: a 4–7% aliquot of HS1 was anomalously present in the nucleus of leukemic cells. When primary CLL cells were in vitro treated whith dexamethazone, cyclosporin A, chlorambucil, or fludarabine the HS1 levels decreased correlating with the sensitivity of these cells to undergo apoptosis. Using a polyclonal antiserum against HS1 a major cleavage product of the apparent molecular weight of 64 KDa and one minor product of approximately 46 Kda was detected in B CLL cells cultured for 24 hours whith drugs. These findings suggest that HS1 plays a pivotal role in the regulation of cell survival of leukemic B cells and suggest that HS1 might represent a target for the development of new drugs to be used in vivo in these patients.