Firm T-lymphocyte binding to microvascular endothelium is an essential step during the early phases of inflammatory responses in skin. Dextrans have been demonstrated to affect lymphocyte-EC-interaction in vivo, however, the mechanisms involved remain unknown. Here we examined whether Dextran 40, a complex colloidal macromolecule widely used as an antithrombotic agent, affects T-lymphocyte-adhesion to endothelial cells applying an established microassay using mouse T-lymphoma (TK-1) and mouse endothelioma (eEND.2) cells. In this model TNFα-induced T-cell-EC-binding is established primarily by the interaction of ICAM-1 and VCAM-1 on EC with their counterreceptors LFA-1 and VLA-4 on T-cells. Dextran 40 in therapeutically achievable levels reduced TK-1 adhesion to eEND.2, inhibiting both constitutive, and TNFα-stimulated binding of the T-cells. This effect was not due to cytotoxicity of Dextran 40 on either T-cells or EC. Selective preincubation of eEND.2 or TK-1 revealed that Dextran 40 acted mainly on the T-cells. To explore further the mechanisms by which Dextran 40 interfered with TK-1 adhesion, their LFA-1 and VLA-4 expression was analyzed by FACScan. Quantitative expression of neither receptor was affected by Dextran 40 treatment. However, analysis of TK-1 by confocal microscopy studies suggest that Dextran 40 interferes with activation-dependent LFA-1 and VLA-4 capping and clustering, which are both essential for T-cell-EC-binding. In summary, Dextran 40 inhibits both constitutive and TNFα-induced T-cell-EC-binding by a selective effect on T-cells and thus might be a useful substance for therapeutic intervention in T-cell mediated cutaneous immune responses.