Based on an ultramicroscopic investigation of the psoralen plus black light-induced photodynamic reaction in the psoriatic cell, there is evidence that the primary reaction takes place in the nuclei, which possess abundant nucleic acid. This initial reaction is manifested by a large vacuole situated in the nucleus. As the vacuole gradually enlarges, the nucleus becomes an annex to the vacuole, while the cytoplasm of the cell succumbs to the necrobiotic process.