Compounds of plant origin showing antioxidative activity are important due to their ability to decrease oxidative stress. They also protect against the harmful effects of the free radicals formed as by-products of some metabolic pathways or as external factors. This supports the application of these compounds (e.g., vitamin C) as additives of food, pharmaceuticals (vitamin E, curcumin, etc.), and cosmetics (e.g., ellagic acid). Natural antioxidants are mainly isolated from plant sources, but besides these, novel, effective, and “green” methods of obtaining antioxidative compounds are being investigated. One of the natural antioxidants is resveratrol belonging to the stilbene polyphenol family. It occurs naturally in grape, and is produced by cells in response to fungal infections or a potential stress factor (e.g., tissue damage). The beneficial effect of resveratrol is based on the inhibition of lipid oxidation and cardioprotective properties. The biocatalyzed synthesis of O-methylated derivatives of resveratrol is especially interesting from the point of view of their possible applications as anticancer agents. The fungal species Phanerochaete chrysosporium is known for its ability to produce methyltransferases, and therefore used as a biocatalyst in resveratrol modifications. Increased production of methyltransferase was stimulated in the strain by changing the conditions of both its cultivation and biotransformation. However, such an approach resulted in the conversion of resveratrol into 3,5-dihydroxybenzaldehyde, indicating a partial degradation of the substrate.