Summary. The resistance of several leukaemic and myeloma cell lines (CCRF, L1210, HL-60, KG-1a and RPMI 8226) to VP-16 was found to increase with cell density and to be maximal (3´5-to 39-fold) in plateau phase cell cultures, as measured by clonogenic and MTT assays. Non-transformed con¯uent Flow 2000 human ®broblasts and Chinese hamster ovary (CHO) cells were also ®ve-and 15-fold resistant to VP-16 respectively. The transition from log to plateau phase was accompanied by a drastic decrease in topoisomerase (topo) IIa content in CHO cells and human ®broblasts, while the leukaemic cells maintained constant cellular levels of topo IIa and topo IIb. However, the nuclear topo IIa content was found to decrease as a result of translocation of the enzyme to the cytoplasmic compartment in the leukaemic cells. This was con®rmed by subcellular fractionation experiments, Western blotting analyses and immunocytochemistry studies. The quantity of topo IIa in plateau phase cytoplasmic fractions ranged from 18% in L1210 cells to 50% in HL-60 and 8226 cells, as measured by both immunoblotting and quanti®cation of the label in immuno¯uorescent images. The cytoplasmic fraction from plateau phase cells retained topo II catalytic activity, as measured by the decatenation of kinetoplast DNA. The nuclear±cytoplasmic ratio of topo IIa may be critical in determining the sensitivity of leukaemic cells to topo II inhibitors. Cytoplasmic traf®cking of topo IIa was observed in plasma cells obtained from patients with multiple myeloma, and perhaps contributes to drug resistance in this disease.