The nucleolar protein B23/nucleophosmin not only participates in pre-ribosome assembly, but is also a key nuclear protein that controls cellular response to apoptotic stimuli. In higher eukaryotes this protein exists in two isoforms, which apparently function in monomeric as well as in homo- or heterooligomeric forms. Data concerning participation of these different molecular forms of nucleophosmin in apoptotic program are very limited and include mainly analysis of the protein monomeric forms. In the present study for the first time we investigated both monomeric and oligomeric forms of nucleophosmin in HeLa cells treated with apoptosis inducers, such as tumor necrosis factor (TNF-α), emetine, or both. It was shown that 6-hour incubation with TNF-ot or 2-hour incubation with emetine did not induce apoptosis and had not affect the monomeric and oligomeric state of the protein. Prolongation of the emetine treatment up to 6 hours did not cause considerable increase in the number of apoptotic cells, either. However, their number dramatically increased following all incubations with the mixture of TNF-α and emetine. Moreover, some of these cells lost their adhesive properties, detached, and reached terminal stages of apoptosis. In the apoptotic cells several structural alterations of B23 protein were detected. In the fraction of cells, which reached terminal stages of apoptosis, the balance between mono- and oligomeric forms of the B23 protein changed due to depletion of monomeric form and appearance of two additional (as compared to control cells) protein bands in the region of oligomers with lower molecular weight. In addition, nucleophosmin oligomers demonstrated differential resistance to heat. Thermal treatment that was sufficient for the complete dissociation of oligomers in the control cell lysates caused only partial dissociation of oligomers in the lysates of terminal apoptotic cells. The lower molecular weight oligomers proved to be the least resistant to heating. Disassembly of the oligomers was accompanied by the appearance of additional protein band in the area of monomers on electrophoregramms. Thus, these observations suggest that apoptotic cascade can cause not only B23 proteolysis that was described earlier in literature, but also the appearance of new (apoptosis-specific) forms of this protein with distinct structural arrangement and properties.