Allantoinase (allantoin amidohydrolase, EC 3.5.2.5) catalyses the hydrolysis of allantoin to allantoic acid, a key reaction in the biosynthesis and degradation of ureides. This activity was determined in different tissues of French bean plants (Phaseolus vulgaris L.) which were grown under nitrogen-fixing conditions. Allantoinase activity was detected in all tissues analysed, but the highest levels of specific activity were found in developing fruits, from which allantoinase has been purified to electrophoretic homogeneity and further characterized. After diethylaminoethyl (DEAE)-Sephacel chromatography, two peaks showing allantoinase activity were obtained in the chromatographic profile and the corresponding proteins were independently purified. Total allantoinase activity was purified 200-fold, indicating the relevance of this enzymatic activity in French bean developing fruits, with allantoinase representing 0.5% of total soluble protein. Both proteins with allantoinase activity are monomeric with molecular masses of 45 and 42 kDa. The specific activities of the purified proteins were 560 and 295 units mg(-1), which correspond to turnover numbers of 25,200 and 12,100 min(-1), respectively. The two proteins have very similar biochemical properties showing Michaelis-Menten kinetics for allantoin with K(m) values of about 60 mM, with high optimal temperatures; are metalloenzymes; are inhibited by compounds reacting with sulphydryl groups; and are unaffected by reducing agents. All analysed tissues exhibited the two activities responsible for allantoin degradation, although one of them was the main form in leaves (the most photosynthetic tissue) and the other protein was the main form in roots (non-photosynthetic tissue). The allantoinase activity and distribution of both proteins have been analysed during fruit development. For both proteins, the allantoinase activity and distribution pattern were the same in plants growing either under nitrogen-fixing conditions or fertilized with nitrate.