Helanalin, a sesquiterpene lactone antineoplastic agent, is toxic at therapeutic doses in murine tumors. The toxicity has been assumed to be correlated with the binding of the drug to cellular thiol groups. Studies were undertaken to increase the intracellular level of GSH in the liver, kidney and other tissues to eliminate the toxicity of helenalin in vivo. Combination of helenalin 8 mg/kg/day i.p.) with L-cysteine (100 mg/kg/day), beta-mercaptoethanolamine (20 mg/kg/day), 18-beta-glycyrrhetinic acid (15 mg/kg/day), or 4,4'-diaminodiphenylsulfone (10 mg/kg/day) afforded improvement in survival of mice bearing P-388 lymphocytic leukemia. However, other thiol-elevating agents, anti-oxidants, intracellular buffering agents, and cardiac treatment drugs were not effective. Hydrocortisone, Cortef, treatment with helenalin afforded improvement in life expectancy. Reduced glutathione (GSH) and non-protein sulfhydryl (NPS) levels were not reduced in the liver, kidney, or circulating red blood cells (rbc) by helenalin treatment. After three days treatment of mice with helenalin, GSH levels were reduced and NPS levels elevated in P-388 tumor cells. Administration of L-cysteine, beta-mercaptoethanolamine, 4,4'-diaminodiphenylsulfone, or 18-beta-glycyrrhetinic acid alone caused no alteration in liver GSH but elevated NPS levels; P388 cell GSH and NPS levels were lowered. Combination of any of these agents, after three days, with helenalin afforded increases in P-388 cell GSH and NPS levels. This data would suggest that helenalin toxicity is not related to the lowering of GSH or NPS levels in critical tissues of mice.