The present invention relates to a method for ex vivo photodynamic treatment of cells. A cell preparation is incubated in a first medium having 5 μM or less of a photosensitive compound. A population of cells in the cell preparation retains the photosensitive compound, for example, a salt of 2-(4,5-dibromo-6-amino-3-imino-3H-xanthen-9-yl)-benzoic acid methyl ester. The first medium is then replaced with an extrusion medium to remove the photosensitive compound. The cell preparation is then illuminated in the extrusion medium to selectively kill the population of cells retaining the photosensitive compound, thereby creating an illuminated cell preparation.
