After staining with dilute solutions (0.1 mg/ml in distilled water) of commercial carmine, a strong reddish orange fluorescence was observed in nuclei from cell smears and frozen and paraffin tissue sections. Optimal exciting light was 436 nm (violet-blue) or 450-490 nm (blue). Compact chromatin from interphase nuclei, mitotic and meiotic chromosomes and the kinetoplast of Trypanosoma cruzi showed the highest fluorescence, while the basophilic cytoplasm appeared weakly fluorescent. No emission was observed in cartilage matrix, mast cell granules or goblet cell mucin. This selective method could be valuable in microscopic and cytochemical studies on chromatin because the carmine fluorescence is stable and preparations can be dehydrated and mounted permanently without changes in the fluorescence pattern.