Integrin undergoes different activation states by changing its quaternary conformation. The integrin beta hybrid domain acts as a lever for the transmission of activation signal. The displacement of the hybrid domain can serve to report different integrin activation states. The monoclonal antibody (mAb) MEM148 is a reporter antibody that recognizes Mg/EGTA-activated but not resting integrin alpha(L) beta2. Herein, we mapped its epitope to the critical residue Pro374 located on the inner face of the beta2 hybrid domain. Integrin alpha(L) beta2 binds to its ligands ICAM-1 and ICAM-3 with different affinities. Integrin is proposed to have at least three affinity states, and the position of the hybrid domain differs in each. We made use of the property of mAb MEM148 to analyze and correlate these affinity states in regard to alpha(L) beta2/intercellular adhesion molecule (ICAM) binding. Our study showed that Mg/EGTA-activated alpha(L)beta2 can adopt a different conformation from that activated by activating mAbs KIM185 or MEM48. Unlike ICAM-1 binding, which required only one activating agent, alpha(L) beta2/ICAM-3 binding required both Mg/EGTA and an activating mAb. This suggests that alpha(L)beta2 with intermediate affinity is sufficient to bind ICAM-1 but not ICAM-3, which requires a high affinity state. Furthermore, we showed that the conformation adopted by alpha(L)beta2 in the presence of Mg/EGTA, depicting an intermediate activation state, could be reverted to its resting conformation.