Abstract: The gram-negative bacteria Citrobacter freundii, Enterobacter cloacae NCTC10005, Erwinia carotovora NCPPB312, Klebsiella pneumoniae, Proteus vulgaris 1753 and Serratia marcescens HIM 307-2 produced natural outer membrane vesicles under normal growth conditions. The membrane vesicles showed bacteriolytic activities against different gram-positive and gram-negative host bacteria. Different killing potencies were obtained by membrane vesicles of different producing organisms against different recipient host strains. In most of membrane-vesicle-producing strains, the exposure to the β-lactam antibiotic cefotaxime and the aminoglycoside antibiotic gentamicin induced the formation of cefotaxime membrane vesicles and gentamicin membrane vesicles, respectively, larger in size and with higher lytic activities against the susceptible host bacteria compared to those produced under normal growth conditions. But the transmission electron microscopy and the plate assay showed that cefotaxime inhibited the formation of membrane vesicles by E. cloacae NCTC10005. Natural membrane vesicles produced by Serratia marcescens HIM 307-2 and P. vulgaris 1753 recorded the widest killing spectrum compared to other natural membrane vesicles. Cefotaxime membrane vesicles of K. pneumoniae showed the highest lytic potency, while C. freundii membrane vesicles exhibited the least lytic spectrum. The cefotaxime membrane vesicles produced by K. pneumoniae had the largest size of 200 nm followed by natural membrane vesicles of P. vulgaris 1753 which had a size of 125 nm, while the smallest membrane vesicles were formed by Serratia marcescens HIM 307-2 grown under normal conditions. The membrane vesicles produced by different gram-negative bacteria used in this study had spherical shapes and sizes ranged from 30 to 200 nm.