We have identified a detoxifying efflux carrier from Arabidopsis using a functional cloning strategy. A bacterial mutant, KAM3, is deficient in multidrug resistance and does not survive on medium containing norfloxacin. After transformation of KAM3 cells with an Arabidopsis cDNA library, transformants were selected for restored growth on the toxic medium. One cDNA clone that complemented KAM3 encodes a novel protein with twelve putative transmembrane domains and contains limited sequence homology to a multidrug and toxin efflux carrier from bacteria. We named this Arabidopsis protein AtDTX1 (for Arabidopsis thaliana Detoxification 1). A large gene family of at least 56 members encoding related proteins was identified from the Arabidopsis genome. Further functional analysis of AtDTX1 protein in KAM3 mutant demonstrated that AtDTX1 serves as an efflux carrier for plant-derived alkaloids, antibiotics, and other toxic compounds. Interestingly, AtDTX1 was also capable of detoxifying Cd(2+), a heavy metal. Further experiments suggest that AtDTX1 is localized in the plasma membrane in plant cells thereby mediating the efflux of plant-derived or exogenous toxic compounds from the cytoplasm.