A novel monoclonal antibody (Mab) (called 3C9) against a major nucleolar phosphoprotein B23 was used to study B23 qualitative and quantitative alterations in phytohemagglutinin (PHA) -stimulated human peripheral blood lymphocytes in indirect immunofluorescence and Western blots. It was shown that lymphocyte proliferation was accompanied by gradual augmentation of nucleoli and their accumulation of the protein B23 up to 2-fold by 16 h and 40-50 fold by 72 h, as compared with the non-stimulated cells. By parallel immunolabeling with the anti-Ki-67 antibody, it was shown that the early changes of B23 amount and localization occurred before an appearance of Ki-67 protein, a well-known marker of proliferating cells. Our results evidence that antibodies against B23 might be applied for recognition of human peripheral lymphocytes at early stages of their activation for proliferation, preceding the S-phase.