Abstract 1. 1. A simple, semiautomated, ninhydrin colorimetric method for the determination of plasma alpha-amino nitrogen is described. 2. 2. Sulphosalicylic acid is used to remove plasma proteins prior to reaction with an air-stable ninhydrin reagent. 3. 3. The values obtained were consistent with those obtained by summation of the individual amino acid values determined by column chromatography. 4. 4. The coefficients of variation between days and within days, were, respectively, 3.8 and 2.2%.