The effect of reduced glutathione on acute lethal toxicity and urotoxicity induced by cyclophosphamide was studied on both mice and rats. The results of this investigation indicate that reduced glutathione is an effective protective agent against bladder damage from treatment with the alkylating agent. The timing of glutathione administration (IV) with respect to cyclophosphamide treatment influenced the uroprotective efficacy of the thiol compound. A schedule-dependent protective effect of glutathione against acute lethal toxicity of the antitumor drug was also observed. This partial protection was accompanied by a reduction in body weight loss following cyclophosphamide treatment. The therapeutic activity of cyclophosphamide on two experimental tumor systems (L1210 and Gross leukemia) was not impaired by combined treatment with glutathione, even at a relatively high dose of glutathione compared with cyclophosphamide.