Rabbits were immunized with reduced glutathione (gamma-glutamyl-cysteinyl-glycine) coupled to bovine serum albumin by glutaraldehyde or a mixture of glutaraldehyde and formaldehyde. The antisera that were formed were tested qualitatively, by screening them against more than 50 amino acids and peptide conjugates that had been immobilized on cellulose discs (spot test), and quantitatively, by immunogold analysis of test conjugates that had been embedded in an epoxy resin. It was shown that the antisera selectively recognized the reduced and oxidized forms of glutathione and that they did not exhibit any significant crossreactivity with glutamate, cysteine, glycine, gamma-glutamyl-cysteine or cysteinyl-glycine. Immunocytochemistry of Vibratome sections of rat cerebellum suggested that glutathione occurs in glial cells as well as in neurons. This was confirmed by electron microscopic, immunogold cytochemistry of tissue from rat cerebellum that had been freeze-substituted and embedded in Lowicryl under low temperature. Gold particles were concentrated over Golgi epithelial cells and perivascular glial processes, but also occurred over several types of neuronal profile including Purkinje and granule cell bodies, and mossy fibre terminals. At the subcellular level, glutathione-like immunoreactivity was found in the cytoplasmic matrix, mitochondria and nuclei. The immunolabelling intensity was strongly reduced in animals that had been pretreated with buthionine sulphoximine, which is known to depress the level of glutathione by inhibiting gamma-glutamyl-cysteine synthetase. The availability of antisera to glutathione is likely to further our understanding of the physiological and pathophysiological roles of this tripeptide.