Endothelial cell VCAM-1 activates adherent monocytes by clustering their VLA-4 receptors, resulting in the modulation of the inwardly rectifying (I_ir) and delayed rectifying (I_dr) K⁺ currents, hyperpolarization of the cells, and enhanced Ca²⁺ influx (10, 11). The present study was undertaken to test the hypothesis that mAb against VLA-4 (mAbVLA-4) mimics VCAM-1 to cluster VLA-4 integrins which play a key role in signaling an increase in the secretion of the proinflammatory cytokine, interleukin-8 (IL-8). Whole-cell ionic currents and IL-8 secretion from THP-1 monocytes that were incubated on polystyrene, VCAM-1, immobilized mAbVLA-4 or an isotype-matched mAb against CD45 (mAbCD45) were measured. Clustering of VLA-4 integrins with a cross-linked mAbVLA-4, but not a monovalent mAbVLA-4, modulated the K⁺ currents in an identical manner to incubation of cells on VCAM-1. Similarly, cross-linked mAbVLA-4 or VCAM-1 augmented Ca²⁺-mediated IL-8 secretion from THP-1 monocytes and was completely abolished by exposure to CsCl, an I_ir blocker. Thus, VLA-4 integrin clustering by cross-linked mAbVLA-4 mimics VCAM-1/VLA-4 interactions sufficiently to be associated with events leading to monocyte differentiation, enhanced Ca²⁺-mediated macrophage function, and possibly atherosclerotic plaque formation.