DNA samples extracted from Japanese cypress leaf tissues contain isopropyl alcohol-precipitable, high molecular weight compounds, which interfere ELISA for cyclobutane pyrimidine dimers (CPD). Removal of the compounds is achieved by DEAE ion-exchange column chromatography and improves the ELISA responses of the DNA. When extracting DNA repeatedly from the same leaf tissues, the DNA samples show CPD responses which increase with the order in sequential extraction, and hence for a reliable detennination of DNA lesion a thorough extraction of DNA is required. Clearing these two problems it was demonstrated that CPD level was slightly higher in the leaves of trees growing under full sunlight than in those growing under UBV -cut sunlight.