Activity of glycollate oxidase (GO) was inhibited by allantoin in spinách leaves after vacuum infiltration. The GO activity was inhibited by 60.1 % at 1.5 mM allantoin. The activity of catalase was inhibited by allantoin too, both in vitro and in vivo experiments, and accumulation of H2O2 inhibited the GO activity. Mercaptoethanol decreased the inhibitoiy effects induced by allantoin and protected the GO activity. As to the mechanism of the inhibitory elfect, it is possible tiiat allantoin inhibited GO indirectly through its inhibitory eťfect on catalase and subsequent H2O2 accumulation in spinách leaves. This, in tum, oxidized essential sulfhydryl group(s) on GO and inhibited its activity.