The aim of this study was the elucidation of the inhibitory influence of Allium (garlic, onion, leek) and Brassica (cabbage, Brussels sprouts) plants juices, on jack bean urease activity. Concentrations of thiosulfinates, the compounds responsible for the inhibition, were determined in studied materials. The kinetics and mechanism of the inhibitions were investigated. Biphasic, time-dependent courses of the inhibition reactions were observed for all tested Allium and Brussels sprouts from Brassica. The cabbage material caused the monophasic course of the inhibition. In the presence of dithiothreitol, a total reactivation of the inhibited urease proceeded for the tested plants except for the onion. The onion juice modified urease, regained only half of the initial activity. The irreversible contribution was related to the presence of 1-propanethial-S-oxide, cepaenes and zwiebelanes formed in the onion juice. It was found that the thermal processing of the plant juices, results in the decrease of thiosulfinates concentration, as well as the efficiency of urease inhibition.