Using specific antibodies we studied the content of nucleolar SURF-6 protein, which participates in rRNA processing, in mouser spleen lymphocytes activated for proliferation with concanavalin A and compared it with the content of nucleolar nucleophosmin/B23 protein and DNA replication factor PCNA, well-known markers of proliferating cells. Using immunocytochemistry and immunoblotting methods we demonstrate that the concentration of all these proteins increases simultaneously with increasing the proportion of proliferating cells. Unlike nucleophosmin/ B23, SURF-6 protein was not revealed in quiescent lymphocyte nucleoli, while the increase of its level in activated lymphocytes preceded elevation of PCNA level. These observations suggest that nucleolar protein SURF-6 can act as a marker of early T lymphocyte activation for proliferation and that it could participate in cell cycle regulation in mammals. © 2009 Springer Science+Business Media, Inc.