AbstractHuman lymphocytes and Chinese hamster ovary cells were exposed to DNA damaging agents for two cell cycles, and the induced sister chromatid exchange (SCE) rate was compared. CHO cells showed a significant increase in SCE following treatment with bleomycin and vincristine whereas human lymphocytes did not. Both CHO cells and lymphocytes showed an increase in SCE with 5‐bromode‐oxyuridine (BrdUrd) and tritiated uridine (3H‐Urd) but the increase was greater in CHO cells. SCE levels were similar after exposure to proflavine and colcemid did not increase the exchange frequency in either cell type. Apparently CHO cells are more sensitive than human lymphocytes to the actions of bleomycin, vincristine, BrdUrd, and3H‐Urd. SCE analysis in response to mutagens and carcinogens should therefore be based on more than one ce