In 7 non-smoking healthy volunteers, the number of micronuclei (MN) was determined in exfoliated buccal mucosa cells before and after rinsing the mouth with an aqueous 10 ppm solution of 2-trans-hexenal during 3 consecutive days. All individuals showed at least a doubling of the MN frequency during one of the next 4 days. An increase of the mean group MN frequency was observed on the fourth day, becoming significant between the sixth and the seventh day. During the next 2 days, the MN frequency dropped down to nearly the control level. In a second study, 7 other volunteers were examined before and after eating 3-6 bananas per day over a period of 3 days. The bananas contained about 35 ppm of hexenal. Six of the 7 individuals showed at least a doubling of the MN frequency during one of the next 6 days. An increase in the mean MN counts was also observed, but the difference to the control value become non-significant during the test period. The results show for the first time that the flavoring constituent 2-trans-hexenal, which is present in many human foods exerts genotoxic effects on human buccal mucosa cells in vivo.