Cells with spontaneously endoreduplicated chromosomes were observed in tonsillar lymphocyte cultures from the second to the seventh day of culture, although at a low frequency. Cell cycle kinetics leading to endoreduplication were examined by the 5-bromodeoxyuridine (BrdU)-acridine orange method. The diplochromosomes showed four fluorescent patterns depending on the period during which BrdU was incorporated into the DNA. The kinetic data also revealed two populations among the cells with endoreduplicated chromosomes: one with a shorter (36 or 48 h) and the other with a longer cell cycle time. The existance of the latter population was shown by the fact that cell without differentially stained chromatids were observed many hours after BrdU addition (up to 120 h). This suggests that some cells with endoreduplicated chromosomes originate from cells arrested in G2.