Sequential reactions of DNA replication have been biochemically studied in detail in prokaryotes and lower, single-cell eukaryotes. Also in higher eukaryotes, the reaction mechanism of DNA replication has been vigorously studied although the process is so complex that the achievements of the studies in bacteria cannot be simply applied. On the other hand, there is another point of view for studying DNA replication, which can be applied only to higher organisms. It is "visualization" of replication on chromosomes. Many methods for visualizing DNA replication on chromosomes have been developed, and these are being rapidly improved. The methods consist of two basic steps, Bromodeoxyuridine (BrdU) labeling and detection of the labeled DNA. To produce replication bands on chromosomes, BrdU must be incorporated into living cells. Therefore, it is essential to select convenient materials that can be easily labeled with the BrdU solution.