The copolymers P(AM-co-4VP) of acrylamide (AM) and 4- vinyl pyridine (4-VP) were synthesized. They had serial varied composition and molecular weight and then were quaternarized by using dimethyl sulfate. Two series of cationic polyacrylamide of quaternary pyridine salt-type(Quaternary P(AM-co-4VP)'QPAV) had been synthesized, which have different cationic degree and molecular weight respectively. In this paper, the antibacterial property of QPAV was mainly investigated using Escherichia coli (E. coli) as model bacterium. Influences of various factors, such as cationic degree, molecular weight, and pH value on the antibacterial property of QPAV were examined with the method of the plate counting, and the antibacterial mechanism of QPAV was studied using the method of measuring the activity Of beta-D-galactosidase and TTC-dehydrogenase. The experimental results show that QPAV possesses excellent antibacterial property, and the antibacterial ratio of QPAV for bacterium suspension of 10(9)CFU/mL reached 100% under the conditions of concentration of 20mg/L and contact time of 5 min. The higher cationic degree of QPAV, the stronger is the antibacterial property. In a certain range of molecular weight, the higher the molecular weight of QPAV, the better is the antibacterial activity. The isoelectric point of protein of E. coli cell is probably pH = 4.4. When pH > 4.4, the antibacterial activity of QPAV increases as the pH increases, and antibacterial ratio reaches to 100% and remain nearly constant when pH > 5.3. When pH < 4.4, however, the apparent antibacterial ratio increases with the decrease of pH value, and this phenomenon may result from combined effects of QPAV and H+ ions on microbe. The experimental results reveal that the antibacterial effect of QPAV is not only based on a restraining action but also on a sterilization action. QPAV is believed to cause cell death by disrupting cell membranes, which allows release of the intracellular contents. (c) 2006 Wiley Periodicals, Inc.