Negative stain electron microscopy (EM) and adhesion assays show that alpha(X)beta(2) integrin activation requires headpiece opening as well as extension. An extension-inducing Fab to the beta(2) leg, in combination with representative activating and inhibitory Fabs, were examined for effect on the equilibrium between the open and closed headpiece conformations. The two activating Fabs stabilized the open headpiece conformation. Conversely, two different inhibitory Fabs stabilized the closed headpiece conformation. Adhesion assays revealed that alpha(X)beta(2) in the extended-open headpiece conformation had high affinity for ligand, whereas both the bent conformation and the extended-closed headpiece conformation represented the low affinity state. Intermediate integrin affinity appears to result not from a single conformational state, but from a mixture of equilibrating conformational states.