Expression of Bcl-2 related proteins in rat microglial primary culture was examined. At relative low cell densities, serum deprivation caused cell death and nuclei condensation of cultured microglia. Expression of a pro-apoptotic protein, Bax, but not Bcl-2, was increased by the serum deprivation. SB203580, a specific inhibitor of p38MAPK, prevented both the serum deprivation-induced Bax expression and microglial death. Immunochemical staining showed that microglia expressing a high level of Bax were subjected to apoptosis-like cell death. These observations suggest that Bax expression underlies the apoptosis of cultured microglia after serum deprivation.