1. The enzyme arginase in chicken kidney is associated with mitochondria and the mitochondrial membranes must be disrupted to obtain maximum activity. 2. When the membranes were disrupted by sonication, approximately 30% higher 2. When the membranes were disrupted by sonication, approximately 30% higher arginase activity was observed than with the nonsonicated samples. 3. The optimum pH for assay of chick kidney arginase was 9.7-9.8. Prior heat treatment with Mn2+ decreased arginase activity. 4. Highest enzyme activity was obtained by using sonicated preparations and measuring initial reaction velocity during the first 1-2 min of incubation.