Through exposure to melphalan or taxol, four novel multidrug resistant variants of the human nasal carcinoma cell line, RPMI-2650 and human lung carcinoma cell line, DLKP, were established In vitro toxicity tests revealed that the RPMI-2650 taxol-resistant variant exhibited significant resistance to taxol, vincristine, adriamycin and vinblastine. The RPMI-2650 melphalan-resistant variant exhibited significant resistance to melphalan, cadmium chlonde, adriamycin, vinblastine and VP-16. The DLKP long term melphalan-selected variant exhibited significant resistance to melphalan, cadmium chlonde, cisplatin, adnamycm and vinblastine. A similar cross resistance profile was observed in the DLKP pulse selection variant with the exception that this variant displayed no obvious cross resistance to vinblastine Adnamycm distnbution studies revealed decreased intracellular adnamycm accumulation in the RPMI-2650 taxol- and melphalan-resistant variants. However, intense nuclear fluorescence was observed following treatment with verapamil and cyclosporin A Charactensation studies demonstrated that the resistant phenotype in the RPMI-2650 taxol-resistant variant appeared to be mainly P-glycoprotem mediated Protein and mRNA analysis by Western blotting, immunocytochemistry and RT-PCR demonstrated an overexpression of MRP1, cMOAT (MRP2) and MRP3 m the RPMI-2650 melphalan-resistant vanant compared to its parental cell line Sulindac or indomethacin could reverse melphalan toxicity m this vanant Western blotting analysis also showed the overexpression of MRP1 in the DLKP melphalan long term and pulse selection cells, suggesting the involvement o f MRP family members in mediating melphalan resistance m these variants Significant iterations in GST activity and metallothionem expression m these vanants were not observed. Results obtained from invasion assays demonstrated that the RPMI-2650 and DLKP melphalan-resistant /ariants were more invasive than their respective parental cells and the RPMI-2650 taxol-resistant vanant. It ippears that increased expression o f a 2Pi, CX5P1, a 6P) and a,6^4 integrins, decreased expression of a 4p1; itronger adhesion to collagen type IV, lamirun, fibronectin and matngel and expression of MMP-2, MMP-9 md c-myc may all contribute to the high mvasiveness of the RPMI-2650 melphalan-resistant vanant ncreased expression of a 2 and Pi integnn subunits and overexpression of MMP-2 and MMP-9 may be esponsible for the invasiveness of the DLKP melphalan-selected variants Decreased expression of a 2 ntegrin which may result in decreased attachment to collagen type IV, lack of cytokeratin 18 and no letectable expression of any proteases may be responsible for the non-invasiveness of the RPMI-2650 taxolssistant vanant. These results suggest that exposure to melphalan can result in not only a multidrug resistance phenotype but ould also make cancer cells more invasive and metastatic.