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Enantiomers of the isoflavonoid phytoalexins maackiain and pisatin were tested for toxicity to 36 fungal isolates representing 19 species. Nine of these species were pathogens of red clover, which synthesizes (–)maackiain; seven species were reported to be pathogenic on garden pea, which synthesizes predominantly (+)pisatin and minor amounts of (–)maackiain. In general, non-host phytoalexins were more inhibitory than host phytoalexins to growth of these pathogens. In addition, the opposite enantiomer of the host phytoalexin was often more inhibitory to a pathogenic fungus than the normally-occurring enantiomer. There was little correlation between the tolerance of fungi to (+) versus (–)pisatin and differential ability to metabolize the pisatin enantiomers. Nevertheless the data suggest that the production of non-host phytoalexins in transgenic plants might increase resistance of red clover and garden pea to some fungal pathogens.