In the present study we investigated the developmental and post-injury pattern of beta-actin immunoreactivity in rat brain. Our data suggest that beta-actin is higher in microglia-macrophages than in any other central nervous system cell type, including neurons and astrocytes. We also show that beta-actin immunoreactivity is particularly high in ameboid-macrophagic cells, suggesting a role on the plastic changes that these cells experience during maturation or after activation.