THE enzyme DNA topoisomerase II, which removes the catenations formed between the DNA molecules of sister chromatids during replication(1) and is a structural component of chromosome cores(2), is needed for chromosome condensation in yeast(3) and in Xenopus extracts(4-6). Inhibitors of topoisomerase II arrest mammalian cells before mitosis in the G2 phase of the cell cycle(7), but also produce DNA damage, which causes arrest through established checkpoint controls(8). It is open to question whether cells need topoisomerase II to leave G2, or control late-cycle progression in response to its activity. Bisdioxopiperazines are topoisomerase II inhibitors that act without producing direct DNA damage(9); the most potent, ICRF-193, blocks mammalian entry into but not exit from mitosis. Here we show that checkpoint-evading agents such as caffeine override this block to produce abortively condensed chromosomes, indicating that topoisomerase II is needed for complete condensation. We find that exit from G2 is regulated by a catenation-sensitive checkpoint mechanism which is distinct from the G2-damage checkpoint.