H. mammatum produces in culture several acidic metabolites that are very toxic to its trembling aspen (P. tremuloides) host; nonhost species are tolerant. Toxins were partially purified by extraction with solvents, absorption on charcoal, gel filtration, chromatography on silicic acid and DEAE columns, and HPLC [high pressure liquid chromatography]. The relative proportions of the toxic metabolites changed with the age of cultures. Clones of P. tremuloides differed in sensitivity; the most resistant clone tolerated 1000-fold higher concentrations of toxins than did a sensitive clone. Sensitivity/tolerance of stem tissue was correlated with leaf response, as indicated by application to wounded surfaces. For leaf bioassays, drops (10 .mu.l) were applied to a wound on the leaf or to the cut ends of petioles. The latter bioassay, which distributed toxins throughout the leaf blade, was more sensitive. The toxins caused increases in O2 uptake by leaves, followed by necrosis and electrolyte leakage. Toxins of H. mammatum are significant determinants in disease development.