Introduction: Polymer-stabilized metal nanocomposites (MeNP) showed great nanobiotechnological potential. In addition to the widely studied AgNPs, magnetic MeNPs used for targeted drug delivery are of growing interest. Aims: We aimed to perform in vitro evaluation of the antibacterial and anti-cancer properties of low-nanometer-scale size Ag, Ni, and Co nanoparticles obtained for this study via borohydride reduction in situin a hybrid matrix SiO₂-grafted polyacrilamide (SiO₂-g-PAAm); SiO₂ core r_av 7 nm, M_vPAA ~ 800 kDa, and mean particle diameters (d_av) 6.1 (Ag), 2.7 (Ni), and 1.9 (Co) nm (Zheltonozhskaya et al. 2021; 2023; 2025). Methods: The Bacterial strains Escherichia coli (ATCC 25922), Staphylococcus aureus (ATCC 25923), and cancer cell lines B95-8 and Wish were used; MDCK cells were used as the control. Antibacterial activity was studied by serial dilutions and well diffusion methods; the MTT test was used for cell viability assay. Results: Antibacterial activity was inherent to AgNP only. For both microbial strains, the MIC of 11.0 µg/ml was at the level of tetracycline. The cytotoxicity was dependent on Me, but not on the cell type. The highest toxicity against B95-8 and Wish cancer cells was found in NiNP (IC₅₀ 1.9 and 2.5 µg/ml, respectively). CoNP exhibited moderate toxicity (IC₅₀ 6.3 and 5.31 µg/ml), while AgNP showed the least toxicity (IC₅₀ 9.2 µg/ml for both cell lines). In MDCK cells, IC₅₀ 20.6 µg/ml was reached by NiNP only. The matrix contributed to cytotoxicity in B95-8 cells but was not toxic to other cell types. Conclusions: Nanocomposites of low-nanometer-scale size MeNPs in a hybrid polymer–inorganic matrix SiO₂-g-PAAm showed high antibacterial and anti-cancer efficiency and were much less toxic in their control of MDCK cells, which makes them promising for anti-bacterial and anti-cancer applications; however, the dependence of their effectiveness on Me and cell typesis in need of further study.