Murine hybridoma 3C9 producing BCA-PC monoclonal antibodies and selectively staining the nucleoli in various cells, most intensely in actively proliferating cell cultures, was obtained by somatic hybridization of cells. BCA-PC MAb belong to IgM. As a rule, IgM antibodies are virtually unavailable for intracellular antigens, and therefore a special method has been developed for fixation and performance of indirect immunofluorescence with BCA-PC. By its molecular weight (37 and 43 kD) and distribution in the cellular nucleus, the antigen detected by BCA-PC resembles the nucleolar protein B23 located in the nucleolar granules. The content of B23 increases with malignant degeneration of cells and during cell proliferation. It is therefore believed that the new MAbs will help characterize the proliferative status of cells which determines the malignancy of a pathological process.