The expression of cell adhesion molecules of the Ig superfamily (Ig-CAM) were examined on embryonic stem (ES) cells during culture in vitro. ES cells maintained an undifferentiated phenotype when cultured in the presence of leukemia inhibitory factor (LIF) or with fibroblast feeder cells; > 90% of cells reacted positively to an antibody (ECMA-7) that marks undifferentiated ES cells. Using flow cytometry, high concentrations of ICAM-1, VCAM-1, and NCAM antigens were detected on undifferentiated ES cells, but their specific receptors, Mac-1, LFA-1, and VLA-4, were not detected. There was also no class I or II major histocompatibility complex (MHC) antigen expression. The ICAM-1 expressed was functional, since anti-ICAM-1 significantly (p < 0.0001) blocked ES cell-lymphocyte binding. Ig-CAM and MHC-1 expression on undifferentiated ES cells was not up-regulated by treatment of cells with interferon-gamma (IFN-gamma), tumor necrosis factor alpha, or flavivirus infection, agents that up-regulate these molecules in other embryonic cell types. Twelve hours after LIF withdrawal, ICAM-1 and NCAM expression decreased significantly, while VCAM-1 was undetectable. However, morphology and ECMA-7 expression remained unchanged. Similar patterns of expression were seen on ES cells maintained on fibroblast feeder cells. This suggests that LIF or other cytokines may maintain the expression of Ig-CAMs on undifferentiated cells. Differentiation was induced by dimethyl sulfoxide treatment for 14 days. Cells changed from a colony-forming to a monolayer morphology, and approximately 60% of the cell population no longer expressed ECMA-7. In these cells, VCAM-1 was undetectable and ICAM-1 and NCAM had declined to low levels. In these differentiated cells, ICAM-1 and MHC-1 were inducible by IFN-gamma. This study suggests that the pattern of expression of the Ig-CAMs in ES cells may have a role in defining the phenotype of differentiated and undifferentiated cells.