Hepatic L-arginase from the Mus booduga (Gray) was purified and its kinetic characteristics were investigated. The enzyme was not adsorbed on DEAE-cellulose, but was retained on CM-cellulose column at pH 7.2. The Michaelis-Menten constant was 8.3 mM for L-arginine and was independent of pH in the range of 7.5-10.5. L-arginine concentrations as high as 0.4 M did not exert substrate inhibition in the pH range 7.4-10.0. Manganese was required at a concentration of 0.05 M for full activation of the enzyme. L-ornithine and L-lysine inhibited the enzyme competitively with inhibitory constants of 1.9 mM and 3.7 mM respectively. Several properties of the L-arginase from Mus booduga clearly identify it as an enzyme similar to ureotelic basic arginases from mammalian liver.