Meripilus giganteus (Pers.) P. Karst. is a basidiomycete fungus known for its bioactive properties, including antioxidant, antimicrobial, and cytotoxic effects. Although research has largely focused on fruiting bodies, mycelium obtained through in vitro culture offers a sustainable and potentially scalable source of bioactive metabolites. This study aimed to compare the chemical composition and biological activity of extracts from the fruiting bodies and mycelium of M. giganteus. Key compound groups were analyzed using high-performance liquid chromatography (HPLC), and biological activity was assessed through DPPH and ABTS antioxidant assays and MTT-based cytotoxicity testing on human gastrointestinal cancer and normal colon epithelial cell lines. The results revealed distinct metabolite profiles between fungal forms and demonstrated that solvent type strongly influenced extraction efficiency. Cytotoxicity assays indicated moderate activity of both extract types, with some selectivity towards colorectal cancer cell lines. These findings suggest that M. giganteus mycelium may serve as a promising alternative to fruiting bodies for the production of antioxidant and potentially chemopreventive compounds. Further studies are recommended to optimize cultivation and extraction conditions to enhance both metabolite yield and biological activity.