Uptake of skin-photosensitive 8-methoxypsoralen (8-MOP) and 5-methoxypsoralen (5-MOP) by human oral mucosal cells were studied. The mucosal cells treated with 8-MOP or 5-MOP were observed by fluorescence microscopy. Fluorescence intensity was weakened at the nuclei of 8-MOPtreated cells, although it was intensified at the nuclei of 5-MOP-treated cells. These findings are not incompatible with the following two experimental results: 1. DNA interior provides a less polar environment as methanol, and 2. Fluorescence intensity of 8-MOP decreases with decreasing solvent polarity, but that of 5-MOP increases. Results obtained show that 8-MOP and 5-MOP molecules are incorporated to DNA interior, and also indicate that DNA molecule is one of the most important reactive sites for PUVA-therapy.