Both sodium ascorbate and ascorbic acid were tested at millimolar concentrations in the mouse lymphoma L5178Y TK+/- cell for chemically-induced cytotoxicity and the induction of gene mutations at the thymidine kinase locus as detected by increased trifluorothymidine-resistance. Neither chemical caused any dose-related increases in trifluorothymidine resistance, even at toxic levels. Increased hydrogen ion concentration was not itself a contributing factor to ascorbic acid toxicity. Ascorbate toxicity was due to products formed in vitro in the absence of cells via chemical reactions with medium components. The formation or persistence of these toxic substances could be prevented by co-incubation with catalase prior to the addition of L5178Y cells. These results suggest that ascorbic acid would not be a mammalian cell mutagen normal physiological conditions.