Proliferating cell nuclear antigen (PCNA), also called DNA polymerase δ-associated protein, is found in the cells of the proliferative compartment of normal tissues and is essential for DNA replication. It can be recognized by many monoclonal antibodies to various epitopes on the molecule. In this investigation one of these, PC 10, has been used on formalin-fixed, paraffin-embedded, human and rodent gastro-intestinal epithelial tissues to assess numerically the labelling index of PC 10 and to compare it, in the rat liver and gastrointestinal tract, with the S-phase fraction as determined by bromodeoxyuridine (BrdUrd) labelling. The distribution of PC 10-labelled cells was recorded with respect to cell position in the intestinal crypts of man. In tissues where both modes of assessment were used, PC 10 staining in the well-established proliferative compartments was found to be more extensive than that of BrdUrd. The higher labelling index with PC 10 can be explained by its identification of PCNA outside the S phase of the cell cycle and also by the long half-life of PCNA protein in post-proliferative intestinal epithelial cells as they migrate towards the villus. Nevertheless the data suggest PC 10 immunostaining in gastro-intestinal epithelia is an operational marker of cell proliferation which is reproducible, quantifiable and can be performed on routinely processed tissues