This study was aimed to analyse: (1) interactions of selected strains of mycorrhizal fungi with Trichoderma spp. from various soils and with Heterobasidion annosum; and (2) some physiological properties of mycorrhizal fungi that are antagonistic towards saprophytes and the pathogen. Because of the poor health of pine stands on former arable fields, research was initiated to select some strains of mycorrhizal fungi that could protect young pine trees, and at the same time could overcome the microbial resistance caused by the presence of expansive saprotrophic fungi in the soil. This study showed that all the studied mycorrhizal fungi antagonistic towards H. annosum, had also an ability to inhibit the growth of Trichoderma spp. from various habitats. Cytoplasm granulation and release of protoplasts from hyphae of the pathogenic and saprotrophic fungi in the interaction zone, observed under a light microscope, attest to antagonistic properties of the studied mycorrhizal fungi in relation to host fungi in vitro. The greatest changes were observed in cultures of Suillus spp. Additionally, the mycorrhizal fungus Suillus bovinus showed an ability to protect the occupied territory from invasion by the soil fungi isolated from various soils. This was reflected in their growth inhibition and changes in the structure of their hyphae. This mycorrhizal fungus released into the substrate some antifungal compounds, whose activity increased with decreasing pH. This was confirmed by the limited growth of the pathogen and saprotrophs in the presence of the liquid left after the culture of S. bovinus strain 15-4 than in the presence of media of the same pH. The compounds released by S. bovinus strain 15-4 caused disturbances in the morphology of microtubular cytoskeleton and mitochondria, which may be the reason for their impaired function in the pathogenic and saprotrophic fungi. Suillus spp. were characterized by a low enzymatic activity, in contrast to isolates of Amanita muscaria and Laccaria laccata. The activity of enzymes regarded as crucial for the process of digestion of fungal cell walls (chitinase, β-1,3-glucanase, β-glucosidase) and proteases released into the substrate, attests to the potential possibility of using the mycelium of saprophytes as a source of nutrients by some strains of ectomycorrhizal fungi. Synthesis of hydroxamate siderophores by some strains of mycorrhizal fungi may be an effective mechanism for iron extraction and for limitation of the growth of other fungi. The activity of phenolic oxidases (laccases and tyrosinases) of A. muscaria and L. laccata strains 9-12 and 9-1, suggests their protection against the activity of lytic enzymes of the pathogen and saprotrophs. This may explain why mycorrhizal fungi are less negatively affected in co-cultures.