To investigate the fate of nuclear ribonucleoprotein (RNP) during apoptosis, we performed a cytochemical and immunocytochemical study of apoptotic mammalian cells by fluorescence and electron microscopy using specific antibodies which recognize different RNP-associated proteins. Light and electron microscopy showed that during apoptosis nuclear RNPs are rearranged, with the formation of fibrogranular heterogeneous clusters which are extruded from the nucleus into the cytoplasm, and finally released at the cell surface, as apoptotic blebs. Restructuring and extrusion of nuclear RNPs apparently determine the arrest of RNP maturation, thus effectively blocking protein synthesis in apoptotic cells.