Summary. When temperature-sensitive (ts) mutant lines of mouse L-cells, ts A1S9 and ts C1, are shifted from 34°C to 38.5°C, a rapid inhibition of DNA synthesis and mitosis occurs. During this phase, cell and nuclear growth continues and results in a substantial increase in cell and nuclear volume. Such cellular modifications are also associated with a marked dispersal of the condensed chromatin masses of interphase nuclei, so that after 48–72 h of incubation at 38.5°C, nuclear profiles of both ts cell lines bear a striking resemblance to the nuclear features characteristic of megaloblastic anaemia. Despite these marked alterations in nuclear chromatin organization, morphometric analysis indicates that the volume of condensed chromatin does not decrease. Current biochemical, cytological and morphometric data on the two ts lines of mutant mouse L-cells during expression of the mutation, suggest that they might provide a useful model to further elucidate cytological features of megaloblastic anaemia.