The heartwood of pine trees contains considerable amounts of the stilbenes pinosylvin and pinosylvin monomethyl ether. In young plants of Pinus sylvestris, however, amounts of these stilbenes are hardly detectable. At the onset of stress conditions, such as UV light or fungal attack, stilbene formation is greatly increased and pinosylvin becomes the preferentially formed phenol. It is considered to be the main phytoalexin produced by pines and may be responsible for induced resistance. A unique synthase acting on cinnamoyl-CoA is the key enzyme for the formation of the stilbenes. Stilbene synthase (pinosylvin-forming) was purified to near homogeneity from 4-wk-old plants treated with UV light. The purification included affinity chromatography on red agarose, molecular sieving, and chromatofocusing. In addition to malonyl-CoA as substrate, stilbene synthase uses cinnamoyl-CoA six times more efficiently than p-coumaroyl-CoA. The stilbene synthase preparation was virtually free of chalcone synthase activity. The enzyme is a homodimer with a subunit molecular weight of 43,000. Separation by chromatofocusing indicates the presence of multiple forms differing in isolelectric points between pH 5.1 and 4.6. Treatment of pine seedlings with Botrytis cinerea leads to a 38-fold increase in stilbene synthase activity within 1 day. This is a rapid response and can be measured prior to any detectable lesion, blight, or sclerotia.