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Aspartate transaminase is protected from the inhibitory action of p-chloromercuribenzoate (p-CMB) by the presence of α-ketoglutarate. The protection occurs over a wide pH range. It has its highest value at pH 5.5 and decreases gradually as the pH increases. The keto substrate exerts its protective effect at concentrations as low as its Km value. An alteration in the amount of the titratable sulfhydryl groups of transaminase by p-CMB in the presence of a-ketoglutarate was found. No change in the spectrum of the enzyme was observed when it was attacked by p-CMB in the presence of α-ketoglutarate. L-Aspartate increases markedly the inhibitory effect of p-CMB upon the aspartate transaminase, when the enzyme, substrate and inhibitor are incubated at pH 5.5 or 10.0. Protection was found when the incubation took place at pH 8.0. The effect of aspartate appears at concentrations as low as its Km value. The inhibitor affects profoundly the spectrum of the product transaminase-aspartate or transaminase-erythro-DL-β-hydroxyaspartate. The titratable amount of the sulfhydryl groups of transaminase by p-CMB changes when the titration takes place in the presence of aspartate. [ABSTRACT FROM AUTHOR]