A new sensitive, accurate and reproducible method has been developed for the assay of L-aspartase (aspartate ammonia-lyase, EC 4.3.1.1) activity. Ammonium ion selective elctrodes were used to measure the increase in activity of ammonium ion produced during the deamination of L-aspartate, a reaction catalyzed by L-aspartase. The sensitive electrode membrane was prepared by immobilizing nonactin in polyvinyl chloride (PVC). Our potentiometric studies indicate an optimum substrate concentration of 0.05 M for L-aspartate and an optimum pH of 8.4. Also, a linear calibration curve of the electrode response towards enzyme activity was obtained in which activity units as low as 0.007 units/ml were detected. These findings were confirmed using spectrophotometric techniques. The increase in absorbance at 240 nm was used to measure the increase in fumarate during the deamination of L-aspartate. The interference from potassium ion was eliminated by dialyzing the enzyme preparation against water.