The recently discovered CLECSF6 protein displays the features of a receptor involved in the down-modulation of leukocyte activation. Although CLECSF6 has been the focus of the interest of many researchers lately, a Western blot characterization of the protein is still lacking. This highly reduces our ability to gain full knowledge of the biological relevance of this protein in cell responses. We produced two rabbit polyclonal antisera that detected a glycosylated protein at approximately 35 kDa in neutrophils. Four different CLECSF6 mRNA species have been discovered to date. When deglycosylated, the protein displayed the molecular weight expected for the longest CLECSF6 form. Neutrophil membrane fractions were strongly enriched in the protein. We showed a down-modulation of the expression of this protein in neutrophils treated with granulocyte-macrophage-colony stimulating factor (GM-CSF), tumor necrosis factor (TNF-alpha), lipopolysaccharide (LPS), and interleukin (IL)-4. This work supports the hypothesis that CLECSF6 is involved in the control of inflammation in neutrophils. (C) 2003 Elsevier Inc. All rights reserved.