In this work, the physicomechanical, chemical and thermal properties of low-density polyethylene (LDPE) modified with a biocide containing metal nanoparticles: nanoAg, nanoCu, and nanoFe2O3 were examined. The presented studies, apart from the previously proven antimicrobial effectiveness, have shown that the application of the biocide in concentrations of 0.5% and 1% has the least influence on the remaining properties. In the remaining concentrations (2–5%), despite the lack of significant influence on the chemical structure, the biocide primarily influenced the strength properties and melt flow rate (MFR). Previous research showed almost 100% antimicrobiological properties against bacteria and fungi of samples with the addition of 4% biocide. For such samples, the presented studies showed a decrease in MFR by approx. 17%, tensile strength at break by approx. 45%, an increase in elongation at breaking by approx. 25%, a decrease in density >2%, no increase in water absorption, and no significant changes in the chemical structure and in thermal properties in relation to LDPE without biocide.