Immunocytochemistry of cysteine sulfinate decarboxylase was performed with a new rabbit antiserum that we have recently produced and characterized using as antigen an 11,000-fold purified fraction isolated from rat liver. This antiserum precipitated cysteine sulfinate decarboxylase enzymatic activity, labeled one band (mol. wt 51,000) on immunoblots of crude tissue extracts and did not stain any cells in peripheral tissues devoid of cysteine sulfinate decarboxylase. According to these criteria, this antiserum appeared to be specific for cysteine sulfinate decarboxylase. Numerous cells were immunolabeled in the cerebellum and the hippocampus. Most notable was the labeling of the small cells surrounding the Purkinje cells and sending radial fibers up to the pial surface of the cerebellar cortex or the staining of small star-shaped cells with thin immunolabeled processes abutting on blood vessels. Identified nerve cells such as the Purkinje cells and granule cells in the cerebellum or the pyramidal and granule cells in the hippocampus were devoid of any immunoreactivity. simultaneous double immunofluorescence was carried out using anti-glial fibrillary acidic protein or anti-S-100 monoclonal antibodies. Cysteine sulfinate decarboxylase as well as glial fibrillary acidic protein- or S-100-immunopositive cells were plotted independently for the same section. Quantitative analysis of the maps indicated that the overwhelming majority of cysteine sulfinate decarboxylase-immunolabeled cells were positive for the established astrocytes markers, glial fibrillary acidic protein or S-100. Between 82 and 98% of cysteine sulfinate decarboxylase-immunolabeled cells were also glial fibrillary acidic protein-positive, depending upon the layer. Cysteine sulfinate decarboxylase immunostaining was localized within the cytoplasm, while that of glial fibrillary acidic protein was linked to the cytoskeleton. Since both labels could not be fully superposed, some double immunolabeled cells may have escaped our analysis. More than 94% up to 99% of cysteine sulfinate decarboxylase-immunolabeled cells were simultaneously S-100-immunopositive. Our quantitative data establish that cysteine sulfinate decarboxylase is strictly localized in astrocytes in the cerebellum and in the hippocampus. This finding suggests that taurine is synthesized by astrocytes in the brain and accordingly may play a role in relation to glial function, possibly within the framework of glial-neuronal interactions.