The distribution of proliferating cells in the digestive gland of the common marine mussel, Mytilus galloprovincialis Lmk, was investigated by means of immunochemical techniques employing PC10, a commercial monoclonal antibody to the proliferating cell nuclear antigen (PCNA). Immunoblot analysis of digestive gland whole homogenates revealed a single crossreactive band of 36-37 kDa, identical to the corresponding protein of rat liver and murine melanoma cells. A band of slightly higher electrophoretic mobility (34-35 kDa) was found in fish liver. In mussel digestive gland, the samples obtained from young specimens presented a more intense signal for PCNA than in those obtained from old mussels, suggesting that the digestive gland cells of young mussels exhibit a higher proliferative activity. In paraffin sections, PC10 specifically labelled nuclei of all cell types, but only a smaller number of cells lining the different digestive epithelia. PCNA expression was more intense in digestive cells than in basophilic cells. Hemocytes circulating along the interdiverticular spaces also presented immunoreactive nuclei. Electron microscopy revealed a specific and moderate PC10 labelling in nuclei. Thus, single gold particles appeared disseminated throughout the nuclei with accumulations of particles in the sites of DNA replication. Taken together, these data reveal that the capacity to proliferate resides within all cell types in the digestive diverticula and do not support the hypothesis of the existence of one stem cell in this epithelium. As opposed to the hepatopancreas of the crab, Carcinus maenas, where mitotic figures and PCNA immunoreactivity are only observed in the embryonic cells within the distal portions of the digestive diverticula, apparently there are not discrete regions of cell proliferation in the digestive gland of mussels.