Raw Pacific whiting (Merluccius productus) treated with hydrogen peroxide or potassium bromate was tested for mutagenicity by the Ames Salmonella/microsome assay with strains TA 98 and TA 100 without and with S-9 activation. To examine the effects of long-term exposure, cooked whiting treated with hydrogen peroxide and stored up to 6 months at -26°C was also tested. In contrast to the raw-treated fish, the cooked sample contained 78% catalasereactive peroxide up to 6 months later. For testing, acidic, neutral and basic fractions were obtained by modifying the procedure of Felton et al. (Mutat. Res., 1981) to reduce emulsion formation. No extract from either potassium bromate or hydrogen peroxide treatment produced mutagens.