Giemsa stain shows an intense and durable cathodoluminescence (CL) when studied at 20–30 kV with a quartz transmission light pipe–photomultiplier system in a scanning electron microscope. Clear CL images of Giemsa-stained chromatin, nuclei, and chromosomes were recorded at low electron does (approximately 10−5–10−4 C/cm2). Careful and control experiments have suggested that true cathodoluminescence of Giemsa has been recorded. The CL emission of Giemsa is attributable to one of its ingredients, eosin-y, whose bromine molecules apparently act as radiation scavengers.