S100B is a calcium binding protein expressed and secreted by astrocytes. Extracellular S100B stimulates the proliferation of astroglial cells and the survival of neurons. Extracellular signal regulated kinases (ERK) are involved in the transduction of proliferating signals in astrocytes. Here we report that S100B significantly increases the activity of ERK in primary cultures of astrocytes, a result which may be related to previous observations of the effect of this protein on glial proliferation. We further confirm that conversion of S100B to its covalent dimer by oxidation of cysteine residues increases its extracellular activity. Although we cannot exclude S100B involvement in other mechanisms of signal transduction, these results suggest that ERK activity in astrocytes is modulated by extracellular S100B.