In a study of the development of a strong inhibitor for gamma-glutamyl transpeptidase (gamma-GTP), the inhibitory effect of 21 arylboronic acids was determined in the gamma-GTP-mediated hydrolysis of N-(L-gamma-glutamyl)-4-nitroanilide. It was shown that most boronic acids can moderately inhibit the enzyme activity and, in particular, the inhibitory effects of (3-formylphenyl)boronic acid and (3-carboxyphenyl)boronic acid (9% and 12%, respectively) are comparable to that reported for the binary L-serine-plus-borate inhibition system (7%). Furthermore, it was shown that the addition of physiologically nontoxic saccharides is very effective in regenerating the enzyme activity. This effect is attributed to the competitive binding of saccharides with the boronic acid inhibitor to pull it out into the water phase. Thus, this paper offers a novel methodology for inhibition and control of gamma-GTP activity.