Aphidicolin, a specific inhibitor of DNA polymerase alpha, was found to induce high frequencies of endoreduplication in Chinese hamster V79 cells in a dose-dependent manner. The aphidicolin-induced endoreduplication was observed when cells were incubated at 37 degrees but not at 41 degrees. Since it is known that DNA polymerase beta is more thermally labile than is DNA polymerase alpha, the data are consistent with the hypothesis that DNA polymerase beta might be responsible for endoreduplication as was reported in mouse trophoblast cells. From the induced diplochromosomes, it was observed that the two unifilarly 5-bromodeoxyuridine-substituted chromatids are generally paired and located inside, whereas the two bifilarly 5-bromodeoxyuridine-substituted chromatids are flanking outside regardless of the presence of sister chromatid exchange or intradiplochromatid interchange.