Human lung parenchyma contains potent accessory cells to stimulate T cells with many features of dendritic cells (DC), including a strong tendency to form aggregates with T cells. As contact phenomena may be crucial for T-cell activation, phenotypic and functional studies were conducted on adhesion molecules. DC from minced lung were isolated by their loose adherence and by their absence of autofluorescent inclusions by flow cytometry. DC compared with peripheral blood monocytes (Mo) show an increased density of Class I and II major histocompatibility antigens. Unexpectedly, the leukocyte integrins (beta 2 group) are decreased in density on DC. CD11a (LFA-1) and CD11b are less expressed, whereas CD11c is preserved. In contrast, the beta 1 chains are denser on DC with an increase in the alpha 5 chain (CD49e) on their surface. The alpha 4 chain of this later group (CD49d) is only weakly present and its ligand VCAM (INCAM-110) is not detected. DC-induced allogeneic T-cell proliferation is suppressed by antibodies against both the beta 1 and the beta 2 chains with a slight additive effect. Among the adhesins, LFA-3 and ICAM-1 are increased on DC compared with Mo. Antibodies against LFA-3, and to a lesser extent against ICAM-1, block T-cell activation. These data improve our knowledge of the phenotype expressed by DC and provide some clues as to how DC adhere with T cells and may transduce signals for T-cell activation by cell-cell interactions.