The major dehydration product of prostaglandin D2, 9-deoxy-delta 9,delta 12(E)-prostaglandin D2, is a potent cytotoxic compound. Like other cytotoxic prostaglandins, this compound possesses an alpha, beta-unsaturated ketone group to which cytotoxic activity has been attributed. This prostaglandin was found to readily conjugate with glutathione (GSH) in vitro. When 9-deoxy-delta 9,delta 12(E)-prostaglandin D2 was incubated with Chinese hamster ovary or hepatoma tissue culture cells, it was rapidly taken up and was recovered in the cell lysate primarily as a GSH conjugate in which the keto group at C-11 and the delta 12 double bond had been reduced. Identification of the GSH conjugate was accomplished by analysis by fast atom bombardment mass spectrometry following purification by high performance liquid chromatography. This GSH conjugate and its cysteinylglycinyl and cysteinyl metabolites were also identified in the cell culture medium. 9-Deoxy-delta 9,delta 12(E)-prostaglandin D2 inhibited cell proliferation of these two cell lines in a concentration dependent manner. Depletion of intracellular glutathione by treatment with diethyl maleate and buthionine sulfoximine decreased the amount of intracellular conjugated prostaglandin recovered, and significantly enhanced the antiproliferative effect of 9-deoxy-delta 9-delta 12(E)-prostaglandin D2 on the growth of these cell lines in a concentration dependent fashion. We conclude that intracellular GSH may modulate the antiproliferative activity of 9-deoxy-delta 9,delta 12(E)-prostaglandin D2 and, possibly, of other cytotoxic prostaglandins.