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To evaluate whether cells selected for doxorubicin resistance were cross-resistant to tumor necrosis factor, the effects of doxorubicin and recombinant human tumor necrosis factor-α (TNF) on doxorubicin-sensitive (WT) and 40-fold doxorubicin-resistant (40F) MCF-7 cell proliferation were assessed. The median dose (MD) for doxorubicin was 14.5 nM for WT cells and 474 nM for 40F cells. The MD for TNF was 0.18 nM for WT cells, while 40F cells were highly resistant to TNF concentrations up to 60 nM. Doxorubicin and TNF in combination were synergistic against WT cells, but not 40F cells. Glutathione depletion by buthionine sulfoxamine sensitized WT cells threefold to TNF, with no change in their response to doxorubicin, while 40F cells showed a twofold increase in doxorubicin sensitivity, with no apparent change in their resistance to TNF. No significant differences in TNF receptor number, Kd, or capacity for TNF internalization were noted between the two cell types. WT cells produced a single 15 kDa TNF degradation product, while the 40F cells produced three lower molecular weight degradation products. We conclude that cross-resistance to TNF in doxorubicin-resistant MCF-7 cells may be explained in part by altered TNF degradation.