The antitumour agents DACA (XR5000; N-[2-(dimethylamino)ethyl]acridine-4-carboxamide) and TAS-103 (6-[2-(dimethylamino)ethylamino]-3-hydroxy-7H-indeno[2,1-c]quinolin-7-one dihydrochloride) have been shown to inhibit two essential nuclear enzymes in vitro, DNA topoisomerase I and DNA topoisomerase (topo) II. To examine whether DACA or TAS-103 stabilise topo I, topo IIα, and topo IIβ cleavable complexes in human leukaemia CCRF-CEM cells, the TARDIS assay (rapped in gaose NA mmunotaining) was used. This assay can reveal drug-stabilised topo–DNA complexes formed in situ in individual cells. The results showed that both DACA and TAS-103 can stabilise topo IIα cleavable complexes in these cells. Topo IIβ cleavable complexes were also formed, but only at high concentrations of DACA and TAS-103. The effect on topo I was less clear, with TAS-103 showing only low levels of cleavable complex formation and DACA having no detectable effect under these assay conditions. This is in contrast to the purified enzyme cleavable complex assay, where both DACA and TAS-103 poisoned topo I. Although both DACA and TAS-103 show a preference for topo IIα in whole cells using the TARDIS assay, the formation of low levels of topo I or topo IIβ cleavable complexes may still play a role in cell death.