Ascorbate (AscH−) functions as a versatile reducing agent. At pharmacological doses (P-AscH−; [plasma AscH−] ≥≈20 mM), achievable through intravenous delivery, oxidation of P-AscH− can produce a high flux of H 2 O 2 in tumors. Catalase is the major enzyme for detoxifying high concentrations of H 2 O 2 . We hypothesize that sensitivity of tumor cells to P-AscH− compared to normal cells is due to their lower capacity to metabolize H 2 O 2 . Rate constants for removal of H 2 O 2 (k cell ) and catalase activities were determined for 15 tumor and 10 normal cell lines of various tissue types. A differential in the capacity of cells to remove H 2 O 2 was revealed, with the average k cell for normal cells being twice that of tumor cells. The ED 50 (50% clonogenic survival) of P-AscH− correlated directly with k cell and catalase activity. Catalase activity could present a promising indicator of which tumors may respond to P-AscH−.