1. In order to study the antigenotoxic potential of eugenol in humans, ten healthy non-smoking males ingested a daily amount of 150 mg eugenol or the placebo for seven consecutive days. After a washout period of one week, groups ingesting eugenol or the placebo were crossed and received the other treatment for seven consecutive days. 2. On days 8 and 22 blood samples were taken for the assessment of standard clinical biochemical parameters. To study the possible antigenotoxic effect of eugenol, on day 8 and 22 blood samples were collected and exposed in vitro to the established genotoxic agents mitomycin C and vinblastine. After exposure the percentage of cells with chromosome aberrations and micronuclei was determined in cultured white blood cells. On days 8 and 22 paracetamol (500 mg p.o.) was administered as test substance to measure phase-II biotransformation capacity. Glutathione-S-transferase (GST) activities were determined in erythrocytes and blood plasma. 3. No significant differences in the clinical biochemical parameters were detected between the eugenol-period and the placebo-period, indicating that daily administration of 150 mg eugenol for 7 days has no toxic affects. 4. No significant differences on the cytogenetic parameters were found after ingestion of eugenol. Thus, there are no indications for an antigenotoxic potential of eugenol in humans, consuming daily 150 mg eugenol for 7 days. 5. A significant reduction in alpha-class GSTs in plasma (P < 0.05), but not in the other measured biotransformation parameters, was found in volunteers during the eugenol-periods as compared to the placebo-period. This may either reflect GST-inhibition by eugenol or protection against background damage of liver cells by eugenol.