Abstract 1. 1. A novel intensely blue copper protein, “umecyanin”, has been isolated from peel of horseradish root ( Armoracia lapathifolia) in a yield of 0.2–0.4 mg/kg of starting material. In the extraction and initial steps it accompanies the peroxidases, but its final purification is achieved by chromatography on CM-cellulose, DEAE-Sephadex, and Sephadex G-75. 2. 2. The copper content is 0.46 + 0.03% and its molecular weight, as determined by gel filtration, is 14 600, corresponding to 1 Cu/mole. 3. 3. In these properties, as well as in colour and spectrum, it resembles stellacyanin, azurin, and the Pseudomonas blue protein. The rice bran blue protein and umecyanin are approximately equally abundant in their plant sources.