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 10−5–10−4C/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.