The effects of cell age, suspending medium and metal ions on the susceptibility of Escherichia coli O157:H7 to the water-soluble maltose chitosan derivative were investigated. In addition, the leakage of glucose, protein (absorbance at 280 nm) and lactate dehydrogenase (LDH) induced by maltose chitosan derivative in saline solution and deionized water, was examined. Cells of E. coli O157:H7 in the mid-exponential phase (6 h) were most susceptible to the chitosan derivative followed by cells in the late-exponential phase (12 h) and stationary phase (24 h). In addition, it was found that the susceptibility of the test organism to the maltose chitosan derivative was less in saline solution than in deionized water. The viable population of E. coli O157:H7 in deionized water containing the maltose chitosan derivative (500 ppm), was reduced from ca 7.6 log cfu/ml to a non-detectable level after 10 h of incubation at 37 degrees C compared to a viable population of ca 6.2 log cfu/ml noted in the chitosan derivative-containing-saline solution. After cells of E. coli O157:H7 were exposed to the chitosan derivative in deionized water, a marked increase in the levels of glucose, protein and LDH activity was observed in the supernatant of cell suspension compared to cells of test organism exposed to the saline solution containing chitosan derivative. Metal ions were also found to reduce the antibacterial activity of chitosan derivative. Their effectiveness increased at greater concentrations and varied with the kinds of metal ions with Ba(2+) the most effective and Mg(2+) the least effective.