To properly evaluate the role of phytoalexins in plant disease resistance it is useful to know whether or not the pathogens involved are capable of metabolizing and detoxifying these compounds. In studying this question, rather than the mechanism of degradation per sé the specificity of fungi in degrading phytoalexins seems crucial. For several years we have been studying the in vitro breakdown of pterocarpanoid phytoalexins by various fungal pathogens (for recent literature, see Fuchs & Hijwegen 1979; Fuchs et al. 1980a, b). The pterocarpan skeleton with its numbering system is given in fig. la; the most common sites of substitution in the different pterocarpans are the 2,3, and 4-positions in the A-ring, the 8,9, and 10-positions in the D-ring and the 6a-position.