The efficacy of all chemotherapeutic agents is limited by the occurrence of drug resistance. To further understand resistance to topoisomerase (topo) II inhibitors, 50 sublines were isolated as single clones from parental cells by exposure to etoposide or m-AMSA. Subsequently, a population of cells from each subline was exposed to three-fold higher drug concentrations allowing 16 stable sublines to be established at higher extracellular drug concentration. Quantitative aspects of topo I, II alpha, and II beta were studied by Northern blotting in 66 resistant cell lines. Reduced topo II alpha mRNA was observed in 95.5% of resistant cell lines, and median topo II alpha mRNA was lower (38.0 +/- 28.1%) in resistant cell lines in comparison to parental cell lines, while increased topo I and II beta were observed (145.5 +/- 89.3%, and 165 +/- 105.9% of that of parental cell lines). But the expression of the topo I and II beta gene were not correlated to that of topo II alpha. Our findings suggest that reduced topo II alpha mRNA seems to be the most important mechanism of resistance to topo II inhibitors.