The two-year survival rate of patients with small cell lung cancer is less than 10%. The major reason for this poor outcome is the development of drug resistance. Panels of small cell lung cancer cell lines have been established, providing models for the study of drug resistance in this tumour. One such model is the doxorubicin-selected H69AR cell line. H69AR displays the typical multidrug resistance phenotype in that it is cross-resistant to anthracyclines, Vinca alkaloids (e.g., vinblastine) and epipodophyllotoxins (e.g., VP-16). However, H69AR cells do not overexpress P-glycoprotein, the membrane drug efflux pump frequently found on multidrug resistant cells. Some alterations in glutathione levels and associated enzyme activities were found but the data do not support the notion that enhanced drug detoxication is involved in H69AR cell resistance. Fewer drug-induced DNA strand breaks, reduced levels of topoisomerase II, and reduced formation of drug-stabilized DNA/topoisomerase II complexes were observed in H69AR cells. These data implicate topoisomerase II in the resistance phenotype of H69AR cells, but cannot explain H69AR cell resistance to the Vinca alkaloids, which do not have topoisomerase II as a target. Monoclonal antibodies against antigens overexpressed on H69AR cells have been derived and four have been characterized. Immunoscreening of an H69AR cDNA expression library has allowed the identification of one of these antigens as p36 (annexin II), a Ca2+/phospholipid binding protein. Chemosensitizers and novel xenobiotics have been examined for their ability to circumvent the drug resistance of H69AR cells. The limited success of these investigations suggests that innovative approaches may be required. In conclusion, the data obtained with H69AR and other models of small cell lung cancer indicate that multiple mechanisms contribute to drug resistance in this disease.