Although various theories have been proposed to account for the mechanism of cadmium-induced cellular injury, none have received strong, direct support from experimental data. An interference with sulfhydryl groups is one of the proposed mechanisms. The ability of reduced glutathione (GSH) to protect hepatocytes from the toxic effects of cadmium has been investigated. When added just prior to cadmium, GSH has a pronounced protective effect, while an additional 15 min after cadmium resulted in partial protection against the ensuing loss of cellular potassium ion. The protection against loss of aspartate aminotransferase into the medium is pronounced, regardless of whether addition of GSH is at zero time or 15 min after cadmium. Addition of the GSH at either time resulted in reduced levels of cadmium associated with the hepatocytes, and may well account for these protective properties of GSH. Protein-bound sulfhydryl groups showed a small decrease in response to cadmium but only after cell injury had been initiated. These data allow the conclusion that cadmium does not exert its cytotoxic effects by simple, single irreversible interaction with cellular thiol groups. Furthermore, as the added GSH remains extracellular, these data indicate that protection can be afforded at an extracellular level even after intracellular exposure to toxic concentrations of cadmium.