1. A new form of enzymically active jack-bean [Canavalia ensiformis (L.) DC] urease corresponding to an S20,w value of 11·8s and a molecular weight of 260000 was investigated. 2. Conversion of 18s urease (EC 3.5.1.5) into the 12s form depends on both low protein concentration and pH. Above pH5·3 urease exists in the 18s form and below pH4·8 in the 12s form; between these two pH values a 12s–18s rapid-equilibrium process is observed. 3. Comparison of the properties of 18s and 12s urease indicated no major differences. 4. A survey of other good urease sources revealed that the 12s form can also be obtained from soya bean (Glycine soja Sieb. and Zucc. cultivar Biloxi) and the bacterium Bacillus pasteurii (Miguel) Migula, but not from watermelon (Citrullus vulgaris Schrad. cultivar Congo). 5. The 12s forms from jack bean and Bacillus pasteurii did not hybridize.
