Arsenic trioxide (As2O3) induces clinical remission in acute promyelocytic leukemia (APL) with minimal toxicity and apoptosis in APL-derived NB4 cells at low (1 to 2 micromol/L) concentration. We examined the basis for NB4 cell sensitivity to As2O3 to identify experimental conditions that would render other malignant cells responsive to low concentrations of As2O3. The intracellular glutathione (GSH) content had a decisive effect on As2O3-induced apoptosis. Highly sensitive NB4 cells had the lowest GSH and the sensitivity of other cell lines was inversely proportional to their GSH content. The t(14;18) B-cell lymphoma cell line had low GSH levels and sensitivity to As2O3 at levels slightly higher than in APL cells. Experimental upmodulation of GSH content decreased the sensitivity to As2O3. Ascorbic acid and buthionine sulfoxide (BSO) decreased GSH to a greater extent, and rendered malignant cells more sensitive to As2O3. As2O3-induced apoptosis was not enhanced by ascorbic acid in normal cells, suggesting that the combination of ascorbic acid and As2O3 may be selectively toxic to some malignant cells. Ascorbic acid enhanced the antilymphoma effect of As2O3 in vivo without additional toxicity. Thus, As2O3 alone or administered with ascorbic acid may provide a novel therapy for lymphoma.