Lysosomes form an intracellular digestive system comparable to the digestive tract of higher organisms, that mediates the breakdown of complex molecules arising both internally and externally to the cell. The system is active both in the normal turn-over of healthy tissues and in disease. Recent investigations which have thrown much light on these processes have been the subject of excellent reviews by De Duve & Wattiaux (1966) and Straus (1967). It is the purpose of this communication to describe our work on another property of the lysosomal system of cells, namely, the concentration of dyes, drugs and carcinogens. While this property of lysosomes is well known to cytochemists (Allison & Young 1964; Koenig 1963; Robbins & Marcus 1963; Zelenin 1966) it appears not to have been studied at the biochemical level. Fluorescence microscopy has shown that living cells in tissue culture take up acridine orange (3, 6-bis-dimethylaminoacridine hydrochloride), the nuclei acquiring a pale green fluorescence, the mitochondria a green fluorescence, and the lysosomes a brilliant orange fluorescence after incubation with the dye at 37 °C (Robbins & Marcus 1963; Allison & Young 1964).