Integrins are cellular adhesion molecules that interact with ligandt in extracellular matrices or on odl surfines and are involved in divene physwlogicsl and pathological processes. Mab SG31 reacts with blood cells of multiple lineages The antigen it expreued on progenitor and immature ceUt at high levels and is down-regulated during maturation. Mature B cells express the antigen at low levels, however, CDS' Bl cell lines and all lymphocytes from peritoneal cavity, a site in which Bl cells predominate, express the antigen at high levels. In addition, SG31 reacts with hemopoietic cells in fetal liver and thymus. SG31 ünmunoprecipitatei a transmembrane glycoprotein consisting of two subunits (160,000 and 100,000 dahon, respectively) and recognizes the smaller one. Flow cytometric analyses show that the tissue distribution of this antigen is similar to that of the o4 integrin except that SG31 reacts with fewer cells in most rymphoid tissues. SG31 does not react with the two known subunits which associate with o4. Together, the data suggest that SG31 recognizes a novel murine integrin subunit which associates with a/4 in hemopoietic and rymphoid tissues. SG31 inhibits 38-07 cell adhésion to fibrooectiu. When administered in vivo the antibody interferes with lymphocyte trafficking. Its high expression on bone marrow progenitor ceils and CD4 CD8 thymocytes suggests that it plays a role in rympho-hanopoiesis and high expression on ahlymphocytes from peritoneal cavity implicates Us role as a homing or retention molecule in this compartment.