Many clinical trials are being conducted to evaluate the efficacies of potential chemopreventive agents in caner prevention. However, the usefulness of such trials are limited by the long duration, large subjects populations and huge costs required to achieve statistical significance. The limitations can be circumvented if suitable intermediate biomarkers can be identified. The overall objective of the present proposal is to evaluate and validate two putative biomarkers, 8- hydroxy-2-deoxyguanosine (80HdG) and malondialdehyde (MDA), to be used as intermediate biomarkers for easy and quick assessment of potential chemopreventive agents at a reduced cost. 80HdG and the baseline level and the intra-individual variation of 80HdG and MDA in urine and buccal mucosal cells of high risk subjects (heavy smokers),and 2) whether the urinary and tissue levels of 80HdG and MDA can be modulated by intervention agents, such as B-carotene and a-tocopherol, both have been shown to be effective in causing regression of oral leukoplakia. The study will be conducted in 2 cycles; each cycle consists of a 1-month run-in, two 3-month dosing and a 3-month wash-out periods. Twenty-four heavy smokers (ages 45-70) will be recruited for the study during a two- year period. After a 1-month run-in period, the subjects will be randomly assigned (8 subjects/group) to receive either placebo, Beta- carotene (30mg) or alpha-tocopherol (400 IU) per day during the 1st 3- month dosing period. The dosage of the intervention agents will be increased to 60 mg or 800 IU per day during the 2nd dosing period. A total of 12 blood, 12 buccal mucosal cell and 12 urine samples will be collected from each subject for the measurement of 80HdG, MDA, Beta- carotene, and alpha-tocopherol using HPLC. Urinary continue will also be measured to assess smoking status. In addition, food frequency questionnaire and 24-hr dietary records will also be collected. It is expected that the beta-carotene and alpha-tocopherol dosing will decrease the levels of 80HdG and MDA, in a dose-response manner, and that the discontinuation of the intervention agents during the wash-out period will return the levels of 80HdG and MDA to the high baseline levels. This information are essential for our next phase of biomarker validation study, i.e. to investigate whether the modulation of the biomarkers by the intervention agents is correlated with clinical response (e.g. regression of oral leukoplakia) in subjects from all ethnic backgrounds. If all successful, this model will be further developed for assessing potential antioxidant chemopreventive agents.