Heterogeneity within an adriamycin-selected multidrug resistant (MDR) human squamous lung carcinoma, DLKP-A, was examined by establishing and characterising nine clones of the cell line. MDR variants of the same lung cell line were established by selecting with VP-16. The resulting cell lines, DLKP/VP-3 and DLKP/VP-8, were characterised and their resistance profiles and mechanisms of resistance were compared with the DLKP-A cell line. All the subpopulations of DLKP-A were found to possess an MDR profile, but their resistance to adriamycin spanned a 9-fold concentration range. The VP-16 selected cells were also MDR. All the cell lines were resistant to vincristine, adriamycin and VP-16 but not to 5-fluorouracil and the VP-16-selected cells were hyper-sensitive to cisplatin. The rank order of resistance was similar in all the cell lines. Heterogeneity was observed in the DLKP-A cell line pertaining to the cells’ drug- and radiation-sensitivity, cell doubling time and biochemical alterations associated with the acquisition of an MDR profile. MDR, induced by the exposure of DLKP cells to adriamycin, did not confer cross-resistance to radiation. Alternatively, irradiation of the DLKP cell line did not result in an MDR cell line. The DLKP-A clones could be mixed to generate a cell population with a toxicity profile similar to the original DLKP-A population. All the MDR cell lines exhibited biochemical alterations; P-glycoprotein expression was increased and mdrl mRNA levels were overexpressed in the MDR cell lines. Adriamycin accumulation by VP-16-selected cells was reflective of their P-glycoprotein levels and was completely reversible by the circumventing agents, verapamil and cyclosporin A. These circumventing agents also enhanced the toxicity of chemotherapeutic drugs to DLKP variants. In addition, topoisomerase II levels were altered in the drug resistant cell lines. A significant decrease in the enzyme level was found in DLKP/VP-8 and DLKP-A, with other cells exhibiting a slight decrease. The topoisomerase II alterations were due to a decrease in the a subunit and these modifications were confirmed by mRNA analysis. Metabolic co-operation was observed in all the variants of the DLKP cell line. While functional gap junctions were observed within the cell lines, intercellular adriamycin transfer did not occur and in general, gap junction inhibitors did not enhance the cells’ sensitivity to the drug. Sensitivity to chemotherapeutic drugs was found to be a cell densitydependent phenomenon. Conditioned medium from a highly resistant variant of DLKP-A enhanced the adriamycin-sensitivity of a low resistant variant. This suggests that a factor, secreted by the cells and whose secretion and/or production was enhanced by the presence of adriamycin, could modify the sensitivity of the cells to the drug.