Summary Mutagenic activity generated in hamburger during pan-frying is dependent upon both temperature and time, with temperature appearing to be the more important variable. Uniformly prepared frozen hamburger patties (115 g; 19% fat) were fried under carefully controlled conditions at 143°C, 191°C and 210°C. Mutagenic activity assayed with the Ames test was not detected in uncooked hamburger, and in hamburger fried at 143°C mutagenic activity remained low at all times studied (4–20 min). In contrast, frying at 191°C or 210°C for up to 10 min resulted in the generation of considerably higher levels of mutagenic activity. Mutagenic activity in fried hamburgers sold at selected restaurants ranged from very low to moderately high. Evidence is also presented for mutagenic inhibitory activity in uncooked and fried hamburger. Mutagenic inhibitory activity decreased mutagenesis mediated by liver S-9 from normal rats but not from Aroclor 1254-treated rats.