This paper presents the high antibacterial activity of ZnO micro-spheres prepared via solution process using zinc nitrate hexahydrate, (+) L-cysteine, HMT and sodium hydroxide against four pathogenic bacteria Staphylococcus aureus, Escherichia coli, Salmonella typhimurium and Klebsiella pneumoniae. The antibacterial activity of ZnO micro-spheres was studied via nutrient broth medium at different concentrations (5 mu g/ml-45 mu g/ml) of ZnO micro-spheres. Our investigation reveals that the lowest concentration of micro-spheres solution inhibits the growth of microbial strain, which was found to be 5 mu g/ml for S. aureus, S. typhimurium and K. pneumoniae whereas for E. coli, it was calculated to be 15 mu g/ml. The diameter of each micro-sphere lie between 2 and 3 mu m as observed from FESEM and Transmission electron microscopy. The composition of synthesized material was analyzed by FTIR spectroscopy and it shows the band of ZnO at 462 cm (1). Additionally, on the basis of morphological and chemical observations the growth mechanism of ZnO micro-spheres was also proposed. (C) 2012 Elsevier Ltd. All rights reserved.