The antibacterial activity of ZnO nanoparticles has been investigated and presented in this paper. Nanoparticles were prepared via non-hydrolytic solution process using zinc acetate di-hydrate (Zn(CH(3)COO)(2).2H(2)O) and aniline (C(6)H(5)NH(2)) in 6 h refluxing at approximately 65 degrees C. In the presence of four pathogens such as Staphylococcus aureus, Escherichia coli, Salmonella typhimurium, and Klebsiella pneumoniae, the antibacterial study of zinc oxide nanoparticles were observed. The antibacterial activity of ZnO nanoparticles (ZnO-NPs) were studied by spectroscopic method taking different concentrations (5-45 microg/ml) of ZnO-NPs. Our investigation reveals that the lowest concentration of ZnO-NPs solution inhibiting the growth of microbial strain is found to be 5 microg/ml for K. pneumoniae, whereas for E. coli, S. aureus, and S. typhimurium, it was calculated to be 15 microg/ml. The diameter of each ZnO-NPs lies between "20 and 30 nm" as observed from FESEM and transmission electron microscopy images. The composition of synthesized material was analyzed by the Fourier transform infrared spectroscopy, and it shows the band of ZnO at 441 cm(-1). Additionally, on the basis of morphological and chemical observations, the chemical reaction mechanism of ZnO-NPs was also proposed.