Cells of the Chinese hamster line EM9 exhibit an elevated SCE frequency and a decreased proliferative response in bromodeoxyuridine (BrdU)-containing medium. Here, these phenotypic features were examined experimentally using polyethylene glycol fusion of EM9 and normal human cells. EM9/human hybrids exhibited both normal SCE frequencies and normal proliferative responses in BrdU-containing medium, suggesting that SCE elevation and BrdU hypersensitivity in EM9 have a common molecular basis.