Adipose tissue secretes a variety of proteins into the bloodstream. We have previously reported a novel cDNA, apMl (adipose most abundant gene transcript 1), which is specifically and abundantly expressed in adipose tissue [1]. Primary structure analysis predicted that the apMl gene product possesses significant homology to collagens VIII, X and complement factor Cl q, and we named it adiponectin. In the current study, we ana-lyzed characteristics of adiponectin in vitro and in vivo. Adipo-nectin protein was proved to be secreted into the medium when the cDNA was transfected to COS cells. Anti-adiponectin cross-reactivities were abundantly detected in the human plasma. In solid-phase binding assays, adiponectin specifically bound to collagen types I, Ill and V, which are present in vascular intima. Immunohistochemical analysis revealed that adiponectin was detected in the walls of the catheter-injured vessels but not in the intact vascular walls. These data suggest that adiponectin is a plasma protein produced by adipose tissue and accumulates in vascular walls when the endothelial barrier is injured.