The enzyme amino acid arylamidase [alpha-aminoacyl-peptide hydrolyze (microsomal) EC 3.4.11.2] catalyzes the release of an N-terminal amino acid from peptides, amides or arylamides. This enzyme is important in N mineralization in soils, because it is involved in the release of amino acids bound to the soil organic matter. The detection of arylamidase in soils is reported, and a precise and accurate method is described for its assay. It involves colorimetric determination of the beta-naphthylamine produced when soil is incubated with L-leucine beta-naphthylamide in 0.1 M THAM buffer (pH 8.0) at 37°C for 1 h. The beta-naphthylamine is extracted with ethanol and reacted with p-dimethylaminocinnamaldehyde to produce an azo compound, the absorbance of which is measured at 540 nm. This enzyme has its optimal activity at pH 8.0 and is inactivated at temperatures above 60°C. The Km values of this enzyme in seven surface soils ranged from 0.19 to 0.35 mM. The activation energy values ranged from 30.6 to 49.8 kJ mol-1 for field-moist soils and from 26.2 to 32.4 kJ mol-1 for their air-dried counterparts. The means of Q10 ranged from 1.32 to 1.71 (avg. = 1.44). Treating soils with toluene, formaldehyde, dimethylsulfoxide, HgCl2, or iodoacetic acid inhibited, and autoclaving completely destroyed, the activity of this enzyme in soils. At 5 mumol g--1 soil, arylamidase activity was inhibited in both air-dried and field-moist samples by 18 of 25 trace elements tested; Ag (I), Hg (II), and Cd (II) were the most effective inhibitors. Co (II), Mg (II), Mn (II), B (III) and As (V) activated this enzyme in field-moist soils and their air-dried counterparts, and W (VI) and Mo (VI) activated this enzyme in air dried soils, but inhibited it in the field-moist soils. The activity of this enzyme in soils was significantly correlated with activities of L-asparaginase (r = 0.91; P < 0.001), L-aspartase (r = 0.90; P < 0.001), urease (r = 0.87; P < 0.001), L-glutaminase (r = 0.84; P < 0.001) and amidase (r = 0.39; P < 0.01). Using substrates containing different amino add moieties showed that the activity of arylamidase decreased as follows: alanine > leucine > serine > lysine> arginine = glycine = histidine > proline (not hydrolyzed). The activity of arylamidase was significantly affected by tillage and crop residue placements. Lime application rate (pH range 4.9 to 6.9) significantly affected the activities of 14 enzymes, including arylamidase, involved in C, N, P, and S cycling.