The effect of IR on EC migration and its reversal using anti-SDF-1 antibody (Ab). Conditioned media from EC monolayers was harvested and assayed for EPC chemotactic potential. This migration pattern showed a similar up-regulation to SDF-1 up-regulation after 5 Gy IR stimulation. Maximal migration occurred with a combination of hypoxia and 5 Gy IR. The addition of SDF-1 to basal media resulted in a response similar to that of conditioned media harvested from IR/hypoxia-stimulated cells. The addition of anti-SDF-1 antibody blunted the migration response to media harvested from ECs exposed to 5 Gy IR, hypoxia, and the combination of both stimuli. This confirmed the causal relationship of increased SDF-1 expression and migration response (n = 6).