The soluble form of complement receptor type 1 in human plasma (sCR1) might correspond to the shedding of the receptor by proteolytic cleavage at the cell surface. A new enzyme-linked immunosorbent assay (ELISA) was established to specifically measure membrane-bound CR1 using a rabbit polyclonal antibody against a 19-amino acid peptide corresponding to the C-terminal sequence of the intracellular domain of CR1 (mCR1-ELISA). This ELISA measured CR1 from solubilized erythrocyte membranes, polymorphonuclear leukocytes (PMN), a B lymphocyte cell line and renal podocyte-derived urinary vesicles in a dose-dependent manner. In contrast, and similarly to recombinant soluble CR1 which lacks the intracellular domain of CR1, plasmatic sCR1 was not recognized, suggesting that sCR1 corresponds to an extracellular fragment of whole CR1. In vitro, PMN were shown to release a soluble form of CR1 which was also not recognized in the mCR1-ELISA, and whose size was smaller (5 kDa) than the CR1 of PMN cell membranes. The release of soluble CR1 was highest for PMN and HL60 cells, followed by U937 cells and three different B lymphocyte cell lines, whereas T lymphocyte cell lines did not release soluble CR1. The levels of CR1 gene expression were also higher in PMN compared to remaining blood leukocytes and the different cell lines tested above. Incubation of PMN with formyl-methionyl-leucyl-phenylalanine, tumor necrosis factor-alpha or lipopolysaccharide accelerated the release of soluble CR1, and incubation with granulocyte/macrophage colony-stimulating factor resulted in sustained CR1 gene expression and higher total soluble CR1 release. Our results suggest that soluble CR1 is produced by cleavage of cell surface CR1, and that a large fraction of human plasma sCR1 is cleaved from PMN. The release of sCR1 by leukocytes may play a role in the control of complement activation at sites of inflammation.