In an attempt to understand more directly the molecular mechanisms involved in the cellular response of endothelial cells to Interleukin-1 (IL-1), we have made several cDNA libraries from human umbilical vein endothelial cells (HUVEC) stimulated for 1 h with IL-1 in the presence of cycloheximide. The cDNA libraries were differentially screened with labelled cDNA derived from mRNA isolated from untreated or IL-1 treated HUVEC. Forty cDNA clones induced by IL-1 were isolated and partially sequenced. Thirty-eight of these corresponded to known genes, that is IL-8, ELAM-1, GRO-alpha, GRO-beta, PA-I and I-kB. The last two clones contained an identical insert belonging to a previously unknown gene. The full length cDNA of this new gene was isolated and called PTX3. It encodes for a 42 kD, 381 amino-acid long protein which shows in its 3' half a high degree of homology to all the known members of the pentaxin gene family, including C-reactive protein (CRP) and serum amyloid P component (SAP). PTX3 may represent a novel marker of inflammatory reactions, particularly those involving the vessel wall.