An l-glutamate biosensor modified by cation exchanger membrane on a palladium (Pd) electrode was designed for the purpose of preventing interferences and electrode fouling during the measurement of serum AST and ALT activities. The rate of signal increase obtained by our sensor for the determination of AST and ALT activity was 0.259 and 0.596 nA/min U(-1)l and the response of the sensor to AST and ALT activity were linear over the range of 8-200 and 8-250 Ul(-1), respectively. Both AST and ALT activities could be measured sequentially by injecting the serum into a solution containing l-aspartate and alpha-ketoglutarate. The rate of current increase was relative to AST activity. The activity of ALT was sequentially determined after addition of l-alanine into the solution. The change in the current increase rate after the addition of l-alanine was proportional to the ALT activity. By using the proposed biosensor, the interference of 1mM ascorbic acid was negligible on a dynamical aminotransferase determination when the dynamic data are taken after the steady state of an elevated baseline has been reached. The proposed l-glutamate biosensor provides adequate sensitivity for the measurement of AST and ALT and is expectable to be applied for rapid blood screening of AST and ALT activity in clinical sample.