AbstractA recent study has shown that ramified microglia in the adult rat optic nerve express the ganglioside GD3[Wolswijk Glia 10:244–249, 1994], thereby raising the possibility that some GD3+in the developing rat central nervous system (CNS) belong to the microglial lineage rather than to the oligodendrocyte lineage, as previously thought. To examine this possibility, sections of postnatal and adult cerebellum were doublelabelled with markers for rat microglia [the B4isolectin derived fromGriffonia simplicifolia(GSI‐B4), the ED1 monoclonal antibody (mAb), and the OX‐42 mAb] and anti‐GD3mAbs (the mAbs R24 and LB1). These immunolabellings showed that ramified microglia as well as amoeboid microglia are strongly GD3+in vivo. Moreover, most, if not all, cells that express high levels of GD3in sections of developing cerebellum appear to belong to the microglial lineage. These observations contradict previous suggestions that the strongly GD3+cells in the putative white matter regions of the developing brain are oligodendrocyte‐type‐2 astrocyte (0–2A) progenitor cells; the cells that give rise to oligodendrocytes in the CNS. The present study did, however, confirm that some 0–2A progenitor cells in sections of postnatal cerebellum are weakly GD3+in vivo. Amoeboid microglia are present in areas of the developing cerebellum where newly generated oligodendrocytes are found, suggesting that these cells play a role in the phagocytosis of the large numbers of oligodendrocytes that die as part of CNS development. © 1995