Abstract Strongly acidic cation exchange resin equilibrated with buffer at pH 7.4 is used to adsorb ammonium ions from 3.0 ml. samples of plasma by ion exchange. After washing away of plasma proteins, the ammonium ion is converted to ammonia and released from the resin by raising the pH of the medium. The concentration of ammonia in this solution is proportional to the intensity of the blue color developed by the addition of phenolate. Normal values of 0.45 to 0.80 μg per Milliliter, with a mean of 0.63 μg per milliliter and a standard deviation of 0.10 μg per milliliter, were determined.