The estrogen receptor alpha (ERa) and estrogen receptor beta (ER~) have been demonstrated to be important for immune system regulation and studies have suggested an antiproliferative effect of ER~ in lymphoid malignancies. We have studied the expression of ERa and ER~ in peripheral blood mononuclear cells from patients with chronic lymphocytic leukemia (CLL). Expression of ERa was low, while ER~ was highly expressed in CLL cells. In order to investigate the possible inhibitory effect of ligand-activated ER~ , we treated CLL cells and Mecl cell lines with the selective ER~ agonist diarypropionitrile (DPN) in culture. Treating Mecl cell lines with DPN showed an antiproliferative effect of ER~ agonist by significantly inhibit the growth of Mec 1 cell lines. This suggests that ER~ agonist may be useful in the treatment of CLL.