Fluorescent Markers for Studying Phagosome‐Lysosome Fusion

Lysosomotropic fluorescent aminoacridines such as acridine orange and quin‐ acrine have achieved prominence as markers for studying lysosome‐phago‐ some fusion, especially in macrophages. Experiments described demonstrate that because the aminoacridines traverse biological membranes with facility, they diffuse throughout the system, and ultimately accumulate intra‐ or extra‐ cellularly where they are most efficiently bound. Their presence or absence in phagosomes is therefore not unequivocally indicative of fusion or nonfusion. Alternative fluorescent lysosomal markers are described, and systems defined for which the aminoacridines may probably be used with confidence.