Microglial response factor-1 is a newly isolated microglial gene, which encodes a Ca(2+) binding protein MRF-1 expressed in microglia and macrophages. We induced 1 h of focal cerebral ischemia or 10 min of global cerebral ischemia in the rat, and investigated the expression of MRF-1 immunoreactivity following ischemia. MRF-1 was present in resting microglia and was upregulated in response to microglial activation. MRF-1 was localized to all the cells of the mononuclear phagocyte system (microglia, monocytes, and perivascular cells) that appeared in the ischemic brain.