Checkerboard analysis documents that CCL1 induces chemotaxis of VSMCs. CCL1 (100 ng/mL) was placed in both top and bottom wells, only in the top well (also containing VSMCs), or only in the bottom wells of the chemotaxis chamber. CCL1 in the bottom chamber induced the greatest number of migrating VSMCs compared with the buffer control (*P = .00 003); however, the activity induced by CCL1 in the top chamber was also significantly increased (**P = .003). Comparison of the migration of VSMCs showed a significant difference between having the agonist in the top versus the bottom chamber (P = .001). The migration of cells when the agonist was in both top and bottom wells was not different than the control (not significant [NS]). These findings document that the chemotaxis of VSMCs in response to CCL1 is mainly directed or chemotactic. Error bars represent SD.