(delta) -aminolevulinic acid (ALA) induces the production of very high amounts of porphyrins in Gram-positive and Gram- negative bacteria. Accumulation of the porphyrins in the bacterial cell is a consequence of the high porphyrin production but most of the porphyrins are excreted from the cells into the medium. By fluorescence, measurements of the endogenic and of the exogenic content of the produced porphyrins can be determined. Bacteria loaded by their own accumulated porphyrins can undergo photoinactivation by various light sources. Killing of S. aureus cells by its endogenic porphyrins can be achieved by illumination with intense blue lights or by HeNe laser. E. coli cells loaded with endogenic porphyrins can be photoinactivated by intense blue and red light.