The chromosome replication patterns of the established cell line CHO have been examined and compared with those of normal primary Chinese hamster cells, using the BudR-Hoechst fluorescence technique developed by Latt (1973). The results of this study indicate that the majority of CHO chromosomes replicate normally. There are four clear-cut exceptions to this rule, three of which are most easily explained as undetected or misinterpreted structural rearrangements of the CHO genome. Several other regions may also replicate differently in CHO and normal primary Chinese hamster cells, but uncertainties in the technique do not allow us to confirm this.