Two compounds forming oxygen radicals were isolated from heated starch by monitoring the potency to form 8-hydroxydeoxyguanosine from deoxyguanosine during purification procedures. These compounds were identified as hydroxymethylreductic acid and methylreductic acid. The former compound was mutagenic to Salmonella strains TA100, TA102 and TA104 and the latter compound induced sister-chromatid exchanges in human NL3 cells. Hydroxymethylreductic acid was found to be a novel compound. Considerable amounts of these compounds were detected in various heat-processed foods.