The S100 proteins are a family of calcium-binding proteins found in the central and peripheral nervous systems of vertebrates. S100beta, the most abundant member of this family in the CNS, mediates calcium signal transduction, and shows neurotrophic, gliotrophic and mitogenic actions that influence the development and maintenance of the nervous system. Another member of the S100 family (S100A10) was found to modulate phospholipid turnover by inhibiting the activity of enzyme phospholipase A2 (PLA2). We determined the concentration of S100beta protein in the plasma of 23 medicated schizophrenic patients and 23 healthy controls. S100beta protein accounts for 96% of the total S100 in the brain. Schizophrenic patients showed reduced S100beta concentrations (p=0.003), and this finding was not related to clinical variables or to intake of antipsychotic medication. Decreased S100beta could be related to the findings of increased PLA2 activity and to brain maldevelopment in schizophrenia. These results are discussed further with respect to the role of adenosine in S100beta release.