Antimicrobial agents were khown to interact with DNA gyrase, based on their decreasing activity and concentrated on the effect of cadmium on the biodegradative process performed by the activity of the enzyme were estimated. Cadmium were chosen because they are often found as contaminant. Bacteriostatic and bactericidal activities were measured with wild-type cells and isomerase mutants of organisms for ciprofloxacin, formation of DNA gyrase complexes. Nalidixic acid, norfloxacin and ciprofloxacin, were lethal for cultures growing aerobically, and the bacteriostatic activity of each antibiotic was unaffected by anaerobic growth. However, lethal activity was distinct for each antibiotic with cells treated aerobically with chloramphenicol or grown anaerobically. Nalidixic acid failed to kill cells under both conditions; norfloxacin killed cells when they were grown anaerobically but not when they were treated with chloramphenicol, ciprofloxacin killed cells under both conditions but required higher concentrations than those required with cells grown aerobically. The entry of nalidixic acid into cells of E. coli was not dependent upon protein synthesis. The lethal action of nalidixic acid also was controlled by transfer of treated cells to drug-free medium. Antibiotic activity against Escherichia coli was examined during aerobic growth, aerobic treatment with chloramphenicol, and anaerobic growth. However, lethal chromosome fragmentation, detected as a drop in viscosity in the absence of SDS, occurred with nalidixic acid treatment only under aerobic conditions in the absence of chloramphenicol. The radiation of sublethal dose of 4.0 kGy at rate of  0.75  kGy/min. was shown as ( NS ) non significant result.