AgNORs are nucleolar proteins that interact specifically with silver salts. The size of silver precipitates measured by image analysis (ICM) in cycling cells proved to be inversely proportional to the cell cycle time and provided a significant correlation with prognosis for a large spectrum of cancers. Because ICM is time-consuming and poorly reproducible among laboratories using different imaging settings, this article presents a new approach to AgNOR quantitation based on flow cytometry (FCM). We report that silver precipitates caused a great decrease in the forward scattered light and that this effect was correlated with the AgNOR's relative area as measured by ICM. These results were confirmed by measuring cell lines having different cell cycle durations. Moreover, double staining using APase-Fast red fluorescence to reveal the Ki-67/MIB 1 antigen of cycling cells and silver nitrate to stain the AgNORs was successfully analyzed by FCM. The procedure makes it possible, for the first time, to validly and rapidly compare the growth fraction and cycling speed of partially proliferating cell populations, such as tumors.