An adhesion assay was developed using human umbilical vein endothelial cell cultures and autologous and allogeneic lymphocytes separated from cord blood. Endothelial monolayers cultured in plain or gamma-interferon (IFN gamma)-supplemented medium were cocultured with lymphocytes for 1 h and non-adherent lymphocytes removed by washing. Autologous and allogeneic lymphocytes exhibited significantly increased adhesion with IFN gamma-treated cells compared with untreated controls. The increased adhesion to IFN gamma-treated endothelial cells was significantly inhibited when autologous or allogeneic lymphocytes were pre-treated with saturating amounts of an anti-CD4 monoclonal antibody. The results indicate that IFN gamma enhances lymphocyte binding to endothelium and that the CD4 molecule may be involved in this process. This could be an important mechanism in targetting the migrating of T-helper (Th) cells to areas of chronic inflammation and in antigen presentation by endothelial cells.