10.22376/ijpbs.2019.10.1.p1-12 Volume 8 Issue 1 2017 (January - March) ZnO nanoparticles can be used in wide range of applications including bionano products, electronics, food additives, sun screens, biosensors, etc. However, the comprehensive toxicological impact posed by ZnO nanoparticles still remains clear. In this research work, we report the preparation and toxicological study of ZnO nanoparticles by simple chemical precipitation method in our laboratory. The prepared ZnO nanoparticles were characterized by using X-ray diffraction, SEM and EADX. The nanoparticles were subjected to antimicrobial assay by agar well diffusion method against both Gram positive lessThan i greaterThan (Staphylococcus aureus and Bacillus subtilis) lessThan /i greaterThan and Gram negative bacteria lessThan i greaterThan (Escherichia coli and lessThan em greaterThan Klebsiella pneumoniae). lessThan /i greaterThan lessThan /em greaterThan ZnO nanoparticles have maintained considerably multidrug resistance against the clinical pathogens such as lessThan i greaterThan Staphylococcus aureus lessThan /i greaterThan and lessThan i greaterThan Bacillus subtilis lessThan /i greaterThan (Gram positive bacteria lessThan i greaterThan ) lessThan /i greaterThan and lessThan i greaterThan Klebsiella pneumoniae and lessThan /i greaterThan lessThan i greaterThan Escherichia coli and lessThan /i greaterThan lessThan i greaterThan ( lessThan /i greaterThan Gram negative bacteria).The antibacterial efficiency of the nanoparticle mainly dependent on the size of the particle along-with other characteristics. The study was further extended to assess the lessThan i greaterThan in vivo lessThan /i greaterThan sub –acute oral toxic effect of ZnO nanoparticles in Swiss Albino Mice. The lessThan i greaterThan invivo lessThan /i greaterThan histopathological studies revealed that the mice treated with 500 mg/kgbwt dosage showed moderate toxicity in the central vein of the hepatocytes after 14 days. In conclusion, ZnO nanoparticle was found to show good antibacterial activity but however it causes moderate toxicity in the organs liver and kidney of Swiss Albino Mice after extensive dosage levels. P. SURESH KUMAR AND A.SAMSON NESARAJ Chemical precipitation, ZnO nanoparticles; characterization; antimicrobial studies; sub acute studies 232-237