A new method for the determination of the amino acid arginine utilizes a dual enzyme catalyzed reaction monitored with an ammonium ion selective membrane electrode of the antibiotic type. The technique gives a linear correlation between arginine concentration and electrode response over the 3 × 10−3Mto 3 × 10−5Mrange and a useful, but non-linear, response over a wider range. Major interferences are urea, K+, and NH4+, but Na+does not interfere in the physiological concentration range.