The metallothioneins in vitro are both effective free radical and transitional metal ion scavengers. For this reason we investigated the expression of this protein in the normal CNS and after ischaemia using a monoclonal anti-metallothionein antibody to metallothionein isoforms I and II. Using immunohistochemistry and confocal laser scanning microscopy, we found anti-GFAP and anti-metallothionein were co-localized in astrocytes at the edges of infarcts. Energy dispersive X-ray microanalysis demonstrated high levels of copper and iron in the metallothionein positive reactive astrocytes. On the basis of the in vitro properties of these proteins, the functional importance of metallothionein in reactive astrocytes could be as a free radical scavenger and metal ion chelator.