5-Bromo-2'-deoxyuridine (BrdU) and 5-chloro-2'-deoxyuridine (CldU) were sequentially administered intraperitoneally into mice at 1-hr intervals. After one additional hr, the small intestines were resected, fixed, and embedded in paraffin. In histological sections stained with monoclonal antibody Br-3 reactive to both BrdU and CldU, and CldU antibody reactive only to CldU, three types of staining could be identified in the proliferating zone. Cells with nuclei stained only with Br-3 antibody were estimated to have completed DNA replication during the first 1 hr and were fixed in G(2)/M-phase. Those nuclei were frequently found in apical areas of the simple columnar epithelium of the intestine, whereas other nuclei were located basally in the cells. This observation suggested intracellular movement of cell nuclei in G(2)/M-phase. Identification of cells in early S-phase became possible using these antibodies in combination with DAB and fluorescence stainings. Replication sites in early S-phase nuclei were found to be numerous, whereas in late S-phase they were larger in size and much smaller in number.