Human umbilical vein endothelial cells (HUVEC) were cultured and treated for varying periods with a range of concentrations of tumour necrosis factor-alpha (TNF-alpha). After this treatment the proportion of peripheral blood mononuclear cells (PBMC), previously depleted of plastic adherent cells, capable of binding to the endothelial cells was assessed. Few PBMC bound to HUVEC which had not been pretreated with TNF-alpha but up to 36% bound after pretreatment of the endothelial cells with TNF-alpha for 10 hr at a concentration of 10 U/ml. Phenotypic characterization of the adherent and non-adherent PBMC subpopulations revealed that natural killer (NK) cells (CD16+) and a proportion of memory helper T cells (CD4+ CD45RA-) bound to TNF-alpha pretreated HUVEC but that few naive helper T cells (CD4+ CD45RA+) showed similar binding. Cytotoxicity assays for NK activity were used to analyse functionally the adherent and non-adherent PBMC subpopulations. It was found that the cell subpopulation which did not adhere to TNF-alpha pretreated HUVEC mediated little lysis of K562 target cells. Conversely, the endothelial cell-adherent PBMC subpopulation produced active lysis supporting the phenotypic evidence that NK cells were concentrated within this subpopulation. These results suggest that TNF-alpha has a rapid and profound up-regulatory effect on the expression of adhesion molecules on the surface of HUVEC. Furthermore, it is apparent that these up-regulated adhesion molecules preferentially bind NK cells and a subset of memory helper T cells from the PBMC population.