Both reduced glutathione (GSH) and oxidized glutathione elicited the phytoalexin response in cell-suspension cultures of bean (Phaseolus vulgaris L.) but had no effect in those of alfalfa (Medicago sativa L.). In bean cells, homoglutathione (HGSH) was the predominant soluble thiol and treatment of cells with fungal elicitor resulted in the accumulation of HGSH but not GSH. In contrast, GSH was more abundant than HGSH in unelicited alfalfa cells, and the intracellular levels of both thiols increased in response to fungal elicitor. Treatment of bean or alfalfa cells with L-oxothiazolidine-4-carboxylate, an artificial precursor for GSH biosynthesis, increased intracellular thiols in an analogous manner to that observed following treatment with fungal elicitor, but did not result in elicitation of the cultures. Differences were observed in the initial metabolic fates of exogenously supplied [(35)S]GSH in bean and alfalfa, but our data do not yet provide a basis for explaining how GSH acts as an elicitor. We conclude that the changes observed in intracellular thiols following exposure of cells to fungal elicitor are a consequence rather than a cause of the initial elicitation signal(s).