Several natural phenolic materials were analyzed for gallic acid and were further characterized for dialyzability and flavonoid content. These materials were added to a diet and fed to broiler strain chicks for 4 weeks. The depression in growth caused by these phenolic materials was compared with that of tannic acid on a gallic acid equivalency basis. The activities relative to tannic acid (congruent to 100) were as follows: methyl gallate, 195; valonia, 117; wattle, 114; allepo tannic acid, 107; eucalyptus, 105, myrobalan, 98; tara crystals, 76; mangrove, 72; quebracho, 39; gallic acid, and ellagic acid, O. It was impossible to clearly relate the growth depressing activity of the materials to hydrolyzability or relative content of smaller phenols.