Urease was purified (4126-fold) from Aspergillus niger (NRRL 003) to a homologous enzyme preparation with a specific activity of 1341 mumol min-1 (mg protein)-1. One species of urease was detected in A. niger, with Km = 3.0 mM, native molecular mass 250,000 Da, pH optimum of 8.0 and a high specificity for urea. Hydroxyurea was a strong competitive inhibitor of urease activity, while N-methylurea acted as a weak uncompetitive inhibitor, based on Lineweaver-Burk and Eadie-Hoftstee plots. The activity of urease was enhanced by, but not dependent on, the presence of Na2EDTA, DL-dithiothreitol (< or = 0.1 to 5.0 mM), Ca2+, Ba2+ and citrate (2 to 20 mM). Urease activity was not affected by Na+, K+, Cl-, Br-, acetate or nitrate (2 to 20 mM), but was significantly decreased in the presence of Li+, Ni2+, Mg2+, Zn2+ or I-. Urease activity decreased 26.0% after 30 min at 65 degrees C, and 86.5% and 100.0% after 5 and 1 min at 80 and 100 degrees C, respectively. Urease activity decreased 30.5% after 90 d at 4 degrees C and 21.0% after 28 d at -20 or -80 degrees C.