RNA synthesis has traditionally been investigated by a laborious and time-consuming radiographic method involving incorporation of tritiated uridine. Now a faster non-radioactive alternative has emerged, based on immunocytochemical detection. This method utilizes the brominated RNA precursor bromouridine, which is taken into a cell, phosphorylated, and incorporated into nascent RNA. The BrU-substituted RNA is detected by permeabilizing the cells and staining with certain anti-BrdU antibodies. This dynamic approach yields information complementing that provided by cellular RNA content analysis at a given time and may be of value in studies of cellular activation and gene expression.