At a time after brain injury when removal of debris and secondary cell death were prevalent, many polymorphonuclear neutrophils were observed in injured tissue. Because neural damage could be mediated by activated neutrophils, we tested in vitro the effect of these leukocytes and other blood components on central nervous system cells. At concentrations similar to those present in blood, polymorphonuclear leukocytes resulted in astrocyte detachment from the substrate and aggregation. These neutrophil concentrations affected both epithelioid and stellate HNK1/A2B5-negative (type 1) astrocytes but not hippocampal neurons. Substrate detachment was partially prevented by corticosterone, but not by protease inhibitors or free-radical scavengers. Co-cultures of purified cortical astrocytes with neutrophils (1/20 cell ratio) contained at the beginning of the experiment approximately 93-98% astrocytes with type 1 markers. After 6-8 days co-culture, many stellate cells insensitive to neutrophils seemed to migrate out of the aggregates. About 70% of these resistant cells had immunological markers typical of type 2 astrocytes. The possible relevance of these findings to reactive astrogliosis and secondary neuronal death is discussed.