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Two alternative hypotheses explain the degradation of organics in the Viking Labeled Release experiment on Mars. Either martian soil contains live indigenous microorganisms or it is sterile but chemically reactive. These two possibilities could be distinguished by the use of pure preparations of glucose isomers. In the laboratory, selected eukaryotes, bacteria, and archaea consumed only D-glucose, not L-glucose, while permanganate oxidized both isomers. On Mars, selective consumption of either D- or L-glucose would constitute evidence for biological activity.