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We have investigated the nonlinear characteristics of a microbubble contrast agent Sonazoid R (Nycomed,Norway), including the second, third, 1/2-order, 3/2-order and 5/2-order harmonics. We have measured the 1/2-order subharmonic response to different transmission sound pressures. We have found that subharmonic signals cannot be generated until the acoustic pressure reaches a certain value, which is the most different subharmonic from high harmonics. This result is favourable for the further study of the subharmonic in the bubbly liquid.The 3/2-order ultraharmonic response to acoustic pressure was also measured.