In this work we have described the nucleolus response on inhibition of protein synthesis visualized by labeling with a new monoclonal antibody. An A3 antigen is observed in the form of numerous foci within a nucleolus and as a few foci over chromosomes in mitotic cells. Regions of A3 antigen localization are susceptible to pepsin treatment but are insensitive to RNase A treatment. This fact indicates that A3 antigen is a protein. On ultrastructural level A3 antigen is localized predominately at the periphery of fibrillar centers. Taken together, A3 antigen properties make it possible to suggest that it is a component of RNA polymerase I transcription machinery. A3 antigen has an intriguing property, namely, the ability to migrate from the nucleolus to the nucleoplasm upon the inhibition of protein synthesis with anisomycin, puromycin, and cycloheximide. The results show that localization of the A3 antigen revealed by the new monoclonal antibody may serve as a cytological indicator of the overall protein synthesis in vitro. © MAIK Nauka 2008.