The fluorescent molecule, DiOC6(3), can be used to label the ER in living cultured cells. The labeling procedures are simple and rapid, and in optimum conditions, the staining is bright and clear and bleaches slowly. The main disadvantage of the technique is toxicity. Photodynamic damage is probably the most serious of the toxic effects, because the damage can be relatively rapid and because the extent and nature of the damage during exposure to the light cannot be determined. To lessen the damage, the exposure of cells to light should be minimized. For many applications, it would be best to verify that cell function is normal during the experimental observations.