Anthglutin (1-γ-l-glutamyl-2-(2-carboxyphenyl)hydrazine) and its analogues were synthesized. Their inhibitory activity on γ-glutamyl transpeptidase was measured. 2-Carboxyphenyl derivatives, including anthglutin, showed inhibitory activity, while the other derivatives did not. The 2-carboxyl group seems to be essential for the inhibitory activity.