The interaction between monocytes and endothelial cells plays an important role in normal vascular biology and the pathogenesis of several vascular diseases. In the present study, interactions of freshly isolated human monocytes (Mos) and cultured umbilical vein endothelial cells (ECs) were quantitatively analyzed by a time lapse microcinematographic optical video system in vitro, with reference to Mo locomotion, adherence, and cytokine influence on these processes. The interaction between Mos and ECs was found to be a dynamic process. Without any stimulation of ECs, Mos generally possessed higher binding capacity to ECs and more active motile property than neutrophils and lymphocytes. The binding ratio of Mos to ECs varied from 0 to 8. Mos crawled over the surface of ECs or along the intercellular boundary areas of ECs by extending pseudopodia as long as 60 microns in length to traverse several ECs. Mos migrated into the subendothelium and could cause the disruption of the EC monolayer. In contrast, neutrophils or lymphocytes showed less adhesiveness to ECs and exhibited less dislocation when they moved on the ECs. Pretreatment of ECs with either human recombinant interleukin-1 beta (IL-1 beta) or tumor necrosis factor (TNF) (10-20 U/ml for 4-8 h) significantly increased adhesion rates of both Mos and neutrophils. Furthermore, the increased adhesion of neutrophils to stimulated ECs was accompanied by incremental increases in the rate of cellular movements. These phenomena were found to be associated with activation of EC surface adhesion molecules. In addition, by using the Boyden chamber assay, we found that IL-1 and TNF did not produce any chemotactic activity for Mos with a concentration range of 10(-3) to 10(3) U/ml. These results indicate that, in comparison with neutrophils and lymphocytes, Mos exhibited active adhesive and motive properties even on unstimulated EC surfaces, which could potentially interfere with the integrity of ECs. Upon exposure to cytokine stimulation, ECs increase the expression of adhesion molecules thereby enhancing both adhesion and locomotion of leukocytes. The distinctively higher affinity for binding to cultured ECs of blood Mos and their active motility relative to other circulating leukocytes may have great consequences in various physiological and pathological processes in vivo, including atherogenesis.