Leptospires are a group of bacteria with a unique ultrastructure and a fascinating swimming behavior that cause a number of emerging and re-emerging diseases worldwide called leptospirosis. The unusual form of motility is thought to play a critical role in the infection process. However, the inhibition mechanism of antiserum on the motility of Leptospira to attenuate the infection efficiency is unknown. In this study, effect of antiserum on motility was quantitatively investigated by swimming speed. Relatively low concentration of antiserum was found to inhibit leptospiral motility, suggesting that the basic immunization can affect the infection efficiency. Recovery of motility a few hours later after the addition of antiserum was observed. This raises a hypothesis that Leptospira carries surface molecules bound with antibodies toward the cell end to escape and recovers the motility.