This report shows that an immortalized endothelial cell line (EA.hy 926) is able to substitute for secondary cultures of human umbilical vein endothelial cells (HUVEC) in the leucocyte:endothelial adherence assay. Enriched preparations of blood polymorphonuclear leucocytes, monocytes and resting and activated lymphocytes exhibited similar adherence characteristics to HUVEC and the EA.hy 926 cells. Cytokines such as tumour necrosis factor (TNF) act on endothelial cells to increase their adhesiveness for leucocytes and in this study there was no difference between TNF-treated HUVEC and EA.hy 926 cells in supporting the enhanced binding of leucocytes. The adherence promoting effect of TNF-treated EA.hy 926 cells appears to be dependent upon their endothelial properties since TNF treatment of A549 cells, the permanent human cell line used to generate the hybrid EA.hy 926 cells did not augment lymphocyte attachment. Monoclonal antibodies against CD11a and CD18 inhibited the binding of lymphocytes to untreated and TNF-treated HUVEC and EA.hy 926 cells and ICAM-1 expression was increased on both monolayers following treatment with TNF. The availability of a hybrid endothelial cell line whose adhesive properties are similar to those of recently isolated endothelial cells should benefit the study of factors that govern leucocyte-endothelial cell interactions and be advantageous to the longitudinal investigation of leucocyte adherence under static conditions.