The Ly-6C antigen is expressed in various cell types of the immune system, including macrophages. Using the monoclonal antibody ER-MP20 which specifically recognises Ly-6C, we have investigated whether brain parenchymatous microglia express Ly-6C in vivo as well as in vitro. In brain sections from developing and adult C57/BI mice, all vessels were strongly immunolabelled. Electron microscopic immunohistochemistry showed that the endothelial cells are the cell type expressing Ly-6C. In contrast, we never observed immunoreactivity on microglia; however, microglial cells proliferating in vitro were strongly ER-MP20 positive. These data show that Ly-6C is not a marker for microglia in vivo.