S100 beta is a calcium binding protein expressed primarily by astrocytes in the brain. In initiating studies of the toxic signalling pathways activated by high concentrations of S100 beta, we previously demonstrated that treatment of astrocytes with microM S100 beta results in a potent stimulation of the mRNA level and enzyme activity of inducible nitric oxide (NO) synthase, an enzyme previously implicated in glial pathology. We provide evidence here that NO formation stimulated by S100 beta can lead to cell death in astrocytes, with characteristics defined for apoptosis. Incubation of astrocytes with S100 beta for 48 h results in an increased percentage of astrocytes undergoing apoptotic cell death, as determined with the TUNEL technique, assays of DNA fragmentation and lactate dehydrogenase release. The cell death induced in responses to S100 beta addition correlates with the levels of NO formation, and an inhibitor of nitric oxide synthase attenuates the NO formation elicited by S100 beta, as well as the cell death. Therefore, we propose that S100 beta has the potential to be trophic or toxic. Although S100 beta may be involved in development, homeostasis and repair, chronic overexpression of the protein may mediate toxic responses or even cell death.