We describe a study examining the effects of fried beef consumption on the frequency of micronuclei in RNA-positive polychromatic erythrocytes (PCEs) in humans. 5 splenectomized individuals participated in a 2-week experiment consisting of 3 phases. During the first and last phases the subjects refrained from eating cooked meat for 4-5 days. This was designed to clear their system of mutagens found in cooked-meat products. In the second phase each person ate a similar diet, but also consumed 4 quarter-pound well-done hamburgers per day for 4 days. Erythrocyte-folate levels were also measured for each donor. No association between diet phase and micronucleus frequencies was observed among the 2 subjects with normal levels of folate. However, among the 3 low-folate donors, the frequency of micronucleated PCEs appeared to be associated with diet. Micronucleus frequencies began to increase 1 day following onset of the beef phase, and started to decline 1 day after finishing this phase. These observations are in agreement with erythrocyte maturation kinetics following short-term exposure. A repeat study performed on one of the low-folate donors consisted of two beef phases separated by a vegetarian phase. Beef in one phase was fried (high in mutagenic amino-imidazoazaarenes [AIAs]) while the beef in the other phase was fried after first being microwave-cooked (low in AIAs). Significant increases in the micronucleus frequency were not observed in this experiment, suggesting that AIAs formed during frying were not solely responsible for the induction of micronuclei.