Endothelial cells contain a cell-volume sensitive chloride conductance with biophysical properties similar to those of a ubiquitously expressed chloride current that is activated by cell swelling. The latter current has been associated with the ICln protein (pICln) which may be the chloride channel itself or, alternatively, a channel regulator. We were therefore interested in whether pICln is involved in the endothelial volume-sensitive chloride current. As a first step, we have cloned human pICln and studied its expression at the protein level. Using a polyclonal antiserum raised against human pICln we found a widespread expression of pICln, both in endothelial cells and in other cell lines. A characteristic feature of pICln is its anomalous migration during denaturing polyacrylamide gel electrophoresis. We also demonstrate that bacterially expressed pICln forms homomeric complexes in vitro.