We examined the effects of a stimulatory anti-beta 1 mAb (TS2/16) and different divalent cations on VLA-4-mediated cell adhesion to vascular cell adhesion molecule-1 (VCAM-1), to the fibronectin-derived CS1 peptide, and to larger fibronectin fragments. Using optimal binding conditions (in the presence of mAb TS2/16 and 1.0 mM Mn2+), the levels of VLA-4-mediated adhesion to VCAM-1 and to CS1 peptide were virtually indistinguishable, and half-maximal inhibition of adhesion to both ligands was achieved using similar levels of an anti-alpha 4 antibody. However, using suboptimal adhesion conditions, two critical differences between adhesion to CS1 peptide (or larger fibronectin fragments) and VCAM-1 were consistently observed. First, stimulation by added mAb TS2/16 had a substantially greater effect on adhesion to CS1 than to VCAM-1 and second, Ca2+ was much less able to support adhesion to CS1 than to VCAM-1. These two differences between adhesion to CS1 peptide and to VCAM-1 were most obvious among cell lines which synthesized inactive or partly active VLA-4 but were not obvious for fully active VLA-4. Together, these results not only reveal crucial differences in the mechanisms of VLA-4 binding to its two ligands, but also lead to increased understanding of the variable activation states of VLA-4. The differential ability to utilize Ca2+ displayed by VLA-4 in different states of activation and the activation of inactive or partly active VLA-4 by the addition of Mn2+ both point to divalent cation sites playing an essential role in determining VLA-4 regulation and ligand specificity.