AbstractMammalian cell populations may be enriched for temperature‐sensitive (ts) mutants by the tritiated thymidine (3H‐TdR) suicide procedure (Thompson et al., '71). Such procedures were carried out on the near‐diploid Chinese hamster ovary (CHO) cell line and on a line made “tetraploid” by Colcemid treatment. Clones oftsmutants were obtained from the diploid line, but in spite of repeated attempts, notsmutants were isolated from the tetroploid line. The phenotypes of the new diploidtsmutants recovered were consistent with the particular selection regime