Immunocytochemical electron-microscopic observation indicated that ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) and/or its degradation products are localized in small spherical bodies having a diameter of 0.4-1.2 micro m in naturally senescing leaves of wheat (Triticum aestivum L.). These Rubisco-containing bodies (RCBs) were found in the cytoplasm and in the vacuole. RCBs contained another stromal protein, chloroplastic glutamine synthetase, but not thylakoid proteins. Ultrastructural analysis suggested that RCBs had double membranes, which seemed to be derived from the chloroplast envelope, and that RCBs were further surrounded by the other membrane structures in the cytoplasm. The appearance of RCBs was the most remarkable when the amount of Rubisco started to decrease at the early phase of leaf senescence. These results suggest that RCBs might be involved in the degradation process of Rubisco outside of chloroplasts during leaf senescence.