P revious studies in our laboratory and others have demonstrated in humans and other mammals two isozymes of arginase (AI and AII) that differ both electrophoretically and antigenically. AI, a cytosolic protein found predominantly in liver and red blood cells, is believed to be chiefly responsible for ureagenesis and is the one missing in hyperargininemic patients. Much less is known about AII because it is present in far smaller amounts and localized in less accessible deep tissues, primarily kidney. We now report the application of enzymatic and immunologic methods to assess the independent expression and regulation of these two gene products in normal tissue extracts, two cultured cell lines, and multiple organ samples from a hyperargininemic patient who came to autopsy after an unusually severe clinical course characterized by rapidly progressive hepatic cirrhosis. AI was totally absent (less than 0.1%) in the patient's tissues, whereas marked enhancement of AII activity (four times normal) was seen in the kidney by immunoprecipitation and biochemical inhibition studies. Immunoprecipitation-competition and Western blot analysis failed to reveal presence of even an enzymatically inactive cross-reacting AI protein, whereas Southern blot analysis showed no evidence of a substantial deletion in the AI gene. Induction studies in cell lines that similarly express only the AII isozyme indicated that its activity could be enhanced severalfold by exposure to elevated arginine levels. Our findings suggest that the same induction mechanism may well be operative in hyperargininemic patients, and that the heightened AII activity may be responsible for the persistent ureagenesis seen in this disorder. These data lend further support to the existence of two separate arginase gene loci in humans, and raise possibilities for novel therapeutic approaches based on their independent manipulation.