Monocyte chemoattractant protein-1 (MCP1) is a potent chemokine for monocytes and microglia. Previous studies in our lab showed that plasmin cleaved mouse MCP1 at lysine (K) 104 and the truncated MCP1 (N terminal fragment 1-104) had a higher chemotactic potency. Here we reported the mechanisms underlying the enhanced activity. Plasmin-mediated truncation of mouse MCP1 increased MCP1-CCR2 interaction, promoted Rac1 activation and lamellipodia formation on microglia. Besides chemotactic activity, MCP1 also functions to disrupt the integrity of the Blood-Brain Barrier (BBB). We also showed here that plasmin-mediated truncation of MCP1 is indispensable for its BBB-compromising activity. Binding of MCP1 to CCR2 leads to activation of Ezrin-Radixin-Moesin (ERM) proteins, which link ZO-1 to actin cytoskeleton. MCP1 also induces phosphorylation of myosin light chain, resulting in reorganization of actin cytoskeleton. The acto-myosin machinery then generates the force to pull ZO-1 away from tight junctions, disrupting the integrity of BBB. Furthermore, we found the mouse MCP1 C-terminal fragment (105-148) is inhibitory to human MCP1, which lacks the C-terminal tail. Fusion of the C-terminal tail to human MCP1 decreased its chemotactic potency and abrogated its BBB-compromising activity. Because MCP1-CCR2 axis is the major signaling system to recruit monocyte and microglia, and microglia are one of the key players during central nervous system (CNS) injury, we investigated the role of MCP1-CCR2 system in vivo using the collagenase-induced intracerebral hemorrhage (ICH) model. We found that MCP1<super>-/-</super> or CCR2<super>-/-</super> mice have smaller hematoma early after injury but the recovery is delayed. The hematoma size is paralleled by the water content in the ipsilateral hemisphere, neuronal death at peri-hematoma region, and neurobehavioral deficit. It should be noted that we found accumulation of activated microglia in mice deficient for MCP1 or CCR2 at later time points, probably because of the activation of other chemokines. Thus, it is still unclear whether microglia activation at later time is neuroprotective or neurotoxic, although early inhibition seems beneficial. Therefore, we propose that early inhibition of microglia activation/accumulation (through antagonists of MCP1-CCR2 axis or plasmin) might be of clinical utility in treating diseases with inflammation in the CNS.