To develop a novel toxicity measurement system using the persistent swimming property of magnetic bacteria along an externally applied magnetic field, certain characteristics of Magnetospirillum sp. AMB-1 cells were examined, including their growth pattern, motility, magnetosensitivity, swimming speed, and cell length distribution. In addition, the effect of toxic compounds on the swimming speed was assessed relative to application as a toxicity sensor. With an inoculum of 1.0 x 10(8) cells/mL, the cells reached the stationary phase with a concentration of about 5 x 10(8) cells/mL after 20 h, under both aerobic and anaerobic conditions. The distribution of the cell length did not vary significantly during the growth period, and both aerobically and anaerobically growing cells showed a similar cell length distribution. Although the cells showed similar growth patterns under both conditions, the anaerobically grown cells exhibited higher motility and magnetosensitivity. Actively growing cells under anaerobic conditions had an average swimming speed of 49 mum/s with a standard deviation of 20 mum/s. When the anaerobically growing cells were exposed to various concentrations of toxic compounds, such as l-propanol and acetone, the swimming speed decreased with an increased concentration of the toxic compound. Accordingly, the relationship between swimming speed and toxicity can be used as an effective quantitative toxicity measurement; furthermore, the relative sensitivity of the proposed system was comparable to Microtox, which is commercially available.