Cell adhesion mediated by the interaction between integrin α4β1 and VCAM-1 is important in normal physiologic processes and in inflammatory and autoimmune disease. Numerous studies have mapped the α4β1 binding sites in VCAM-1 that mediate cell adhesion; however, little is known about the regions in VCAM-1 important for regulating soluble binding. In the present study, we demonstrate that 6D VCAM-1 (an alternatively spiked isoform of VCAM-1 lacking Ig-like domain 4) binds α4β1 with a higher relative affinity than does the full-length form of VCAM-1 containing 7 Ig-like extracellular domains (7D VCAM-1). In indirect binding assays, the EC 50 of soluble 6D VCAM-1 binding to α4β 1 on Jurkat cells (in 1 mM MnCl2) was 2 × 10 -9 M, compared with 7D VCAM-1 at 11 × 10-9 M. When used in solution to inhibit α4β1 mediated cell adhesion, the IC50 of 6D VCAM-1 was 13 × 10-9 M, compared with 7D VCAM-1 measured at 150 × 10-9 M. Removal of Ig-like domains 4,5, or 6, or simply substituting Asp328 in domain 4 of 7D VCAM-1 with alanine, caused increased binding of soluble 7D VCAM-1 to α4β1. In contrast, cells adhered more avidly to 7D VCAM-1 under shear force, as it induced cell spreading at lower concentrations than did 6D VCAM-1. Finally, soluble 6D VCAM-1 acts as an agonist through α4β1 by augmenting cell migration and inducing cell aggregation. These results indicate that the domain 4 of VCAM-1 plays a contrasting role when VCAM-1 is presented in solution or as a cell surface-expressed adhesive substrate.