Conjugates of the B₄ isolectin fromGriffonia simplicifolia seeds and horseradish peroxidase were used as a histochemical reagent for the specific visualization of microglial cells in the rat CNS. Resident microglia bearing galactose-containing glycoconjugates were stained throughout the brainstem and cerebellum. In the first week following axotomy of the facial nerve, a profound and rapid accumulation of reactive microglia, as evidenced by increasing lectin reactivity, was seen to take place in the facial nucleus. Light microscopy of paraffin sections demonstrated binding of lectin-horeseradish peroxidase conjugates to microglial cytoplasmic processes. When ultrastructural cytochemistry was performed, reaction product was found localized on microglial plasma membranes, as well as on intracytoplasmic membranes. The glial reaction to axotomy was studied further with double labelling of microglia and astrocytes by lectin histochemistry and immunostaining for glial fibrillary acidic protein, respectively. Our results demonstrate the presence of membrane-associated glycoconjugates containing terminal α-D-galactose residues on microglia, but not on other glial cell types. The possible nature and function of these glycoconjugates are discussed.