Mutagenic activity detected in beef extracts and in fried beef heated for varying periods of time was purified and then analysed by high-performance liquid chromatography in combination with mass spectrometry (LC-MS). The major mutagenic component found in all of the beef products was identified as 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) followed by lesser amounts of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (4,8-DiMeIQx). Identification and quantification of mutagens were achieved by the use of deuterium-labelled analogues. Measured levels of MeIQx and 4,8-DiMeIQx in different batches of beef extract were in the range 11.7-52.2 and 0-11.2 ng/g, respectively, and in beef heated at 275°C for 5-15 min the values of MeIQx and 4,8-DiMeIQx were in the range 2.7-12.3 and 0-3.9 ng/g, respectively. The levels of IQ found in beef extracts were 0-36.8 ng/g and in fried beef the amounts were estimated at 0.3-1.9 ng/g. The method of purification is rapid, requiring only XAD-2 adsorption followed by an acid-base liquid partition against ethyl acetate and blue cotton treatment (trisulpho-copper-phthalocyanine) prior to LC-MS analysis. Because of the sensitivity of LC-MS, mutagens present in cooked beef can be detected at the low parts-per-billion-level and as little as 10 g of cooked beef was required for analysis.