We have investigated the influence of ex vivo expansion of human CD34+ cord blood cells on the expression and function of adhesion molecules involved in the homing and engraftment of haematopoietic progenitors. Ex vivo expansion of umbilical cord blood CD34+ cells for 6 d in the presence of interleukin 3 (IL-3), IL-6 and stem cell factor (SCF) or IL-11, SCF and Flt-3L resulted in increased expression of α4, α5, β1, αΜM and β2 integrins. However, a significant decrease in the adhesion of progenitor cells to fibronectin was observed after the ex vivo culture (adhesion of granulocyte-macrophage colony-forming units (CFU-GM) was 22 ± 4% in fresh cells versus 5 ± 2% and 2 ± 2% in each combination of cytokines). Incubation with the β1 integrin-activating antibody TS2/16 restored adhesion to fibronectin. Transplantation of ex vivo expanded umbilical cord blood CD34+ cells was associated with an early delayed engraftment in non-obese diabetic/severe combined immunodeficient (NOD/SCID) mice. Incubation of cells with the monoclonal antibody TS2/16 before transplantation almost completely abrogated NOD/SCID repopulating ability of both fresh and expanded CD34+ cells. The seeding efficiency of fresh and expanded CD34+ cells was similar, but markedly reduced after incubation with the TS2/16 monoclonal antibody. Our results show that functional activation of β1 integrins could overcome the decreased very late antigen (VLA)-4- and VLA-5-mediated adhesion observed after ex vivo expansion of haematopoietic progenitors. However, in vivo, these effects induced an almost complete abrogation of the homing and repopulating ability of CD34+ UCB cells.