Patterns of drug sensitivities in relation to topoisomerase II gene expression and activity were studied in eight human lung cancer cell lines not selected in vitro for drug resistance. The cytotoxicities of doxorubicin, etoposide, teniposide, cisplatin, camptothecin, and 5-fluorouracil were measured and, remarkably, these unselected cell lines were shown to have a common pattern of multidrug sensitivity, i.e., a multidrug sensitivity phenotype. In fact, drug sensitivities were significantly correlated with each other in the studied cell lines, the correlation being best for the topoisomerase II-targeted agents and cisplatin, less strong with camptothecin, and weak with 5-fluorouracil. Almost 1-log range difference of topoisomerase II gene expression was found in these cell lines, and this was not explained by the cell-doubling time or cell cycle distribution. The level of topoisomerase II gene expression was positively and highly correlated with the cell sensitivity to epipodophyllotoxins, doxorubicin, and cisplatin in seven cell lines. Although weaker, an association was also observed between topoisomerase II gene expression and camptothecin cytotoxicity, while no association was observed with 5-fluorouracil. However, a non-small cell lung cancer cell line with neuroendocrine properties had very low levels of expression of the topoisomerase II gene, despite being highly sensitive to all drugs tested. The levels of topoisomerase I gene expression were not found to be correlated with the cytotoxicity of any drug tested. A specific enzymatic activity assay and a teniposide-stimulated DNA cleavage assay showed that the extent of active topoisomerase II present in nuclear extracts paralleled the level of topoisomerase II gene expression. Furthermore, in addition to the normal transcript, an abnormally sized topoisomerase II message and a rearrangement of the topoisomerase II gene were detected in a poorly sensitive small cell lung cancer cell line. Therefore, low levels of topoisomerase II gene expression, and possibly mutations, may predict a reduced sensitivity of unselected human lung cancer cell lines to several drugs, including agents with a cellular target other than topoisomerase II. It is hypothesized that topoisomerase II might be involved in a common pathway of cell death induced by drugs in tumor cell lines which present a multidrug sensitivity phenotype.